ATMO 200
ATMO 200
Alison Nugent, David DeCou, and
Shintaro Russell
Christina Karamperidou and
Jennifer Small Griswold
ATMO 200 Copyright © by Alison Nugent, David DeCou, and Shintaro Russell.
All Rights Reserved.
Atmospheric Science: ATMO 200
Companion Text
This textbook serves as an introduction to atmospheric science
for undergraduate students and is the primary textbook for the
ATMO 200: Atmospheric Processes and Phenomenon course
at the University of Hawai’i at Mānoa. The book covers basic
atmospheric science, weather, and climate in a descriptive and
quantitative way.
1
Disclaimer
Before using this book, please be aware of the following:
a) the book is currently being piloted at the University of
Hawaiʻi at Mānoa in an undergraduate Atmospheric Sciences
course,
b) there is still a lot of copyediting, images, and narrative to be
included, so as a result
c) chapters will be continually modified, with a more-complete
edition available in mid-2019.
If you have suggestions or comments, please email Dr. Alison
Nugent at ‘anugent@hawaii.edu’.
Thank you!
2
Preface
This open access textbook was developed as an introductory
resource for atmospheric science to reflect the unique weather
patterns in Hawai‘i and the needs of the course to be both
descriptive and quantitative. Its intended audience are students
from the University of Hawai‘i at Mānoa enrolled in the
Atmospheric Sciences 200 course, Atmospheric Processes and
Phenomenon. However, this open access textbook may be of
interest to other courses interested in teaching atmospheric
science at the introductory level. This book is best viewed online
using the Pressbooks webbook format however, multiple
formats (e.g., pdf, epub, mobi) are also available.
3
About the Contributors
This open access textbook was made possible through the
collaboration of faculty, students, and staff at the University of
Hawai‘i at Mānoa working together in the spirit of Aloha.
4
ATMO 200 • 5
Authors
Prof. Alison D. Nugent
Alison Nugent is an Assistant Professor in the Department of
Atmospheric Sciences in the School of Ocean and Earth Science
and Technology at the University of Hawai‘i at Mānoa. She
received her PhD in 2014 from Yale University and holds a
BSc in Earth and Planetary Sciences from Harvard College.
Her research focuses on mesoscale and microscale meteorology,
especially mountain meteorology and cloud microphysics.
Dr. Nugent acted as the primary editor for the textbook including
content, text, and visuals, overseeing the textbook development.
She currently teaches a number of undergraduate courses in
atmospheric science. She is dedicated to developing readily
available and accessible education materials and curricula to
6 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
ensure that her students can relate and easily learn from the
content.
David Decou
David DeCou is one of the co-authors of the first draft of the
textbook. He wrote the first draft of most of the odd
chapters. David Decou holds a BS in Meteorology from
Valparaiso University and an MS from the Department of
Atmospheric Sciences at the University of Hawai‘i at Mānoa,
class of 2018.
ATMO 200 • 7
Shintaro Russell
Shintaro Russell is another co-author of the first draft of the
textbook. He wrote the first draft of most of the even chapters.
Shintaro Russell graduated with a BS in Atmospheric Sciences
in 2017 from the Department of Atmospheric Sciences at the
University of Hawai‘i at Mānoa.
8 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Contributors
Prof. Christina Karamperidou
Christina Karamperidou is an Assistant Professor in the
Department of Atmospheric Sciences at the University of
Hawai‘i at Mānoa. She received her PhD in 2012 from Columbia
University and holds a BS and MS in Civil & Environmental
Engineering from the Aristotle University of Thessaloniki,
Greece. Her research interests include dynamics and
predictability of the El Niño/Southern Oscillation phenomenon,
paleoclimate studies, interactions between tropical and
extratropical American Meteorological Society, cited 2019:
Climate. Glossary of Meteorology. [Available online at
ATMO 200 • 9
http://glossary.ametsoc.org/wiki/Climate.]"
class="glossaryLink">climate variability, and hydro-American
Meteorological Society, cited 2019: Climate. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Climate.]" class="glossaryLink">climate modeling.
Dr. Karamperidou teaches undergraduate and graduate courses
in American Meteorological Society, cited 2019: Climate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Climate.]"
class="glossaryLink">climate
modeling and American
Meteorological Society, cited 2019: Climate. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Climate.]" class="glossaryLink">climate data applications.
She is passionate about training a future generation of scientists
who will aim to decipher the complexity of the American
Meteorological Society, cited 2019: Climate. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Climate.]" class="glossaryLink">climate system while
answering the increasing need for incorporating American
Meteorological Society, cited 2019: Climate. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Climate.]" class="glossaryLink">climate information into
policy and resource management.
10 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Prof. Jennifer D. Small Griswold
Jennifer Small Griswold is an Associate Professor in the
Department of Atmospheric Sciences at the University of
Hawai‘i at Mānoa. She received her PhD in 2009 from the
University of California at Santa Cruz in Earth and Planetary
Sciences and holds BS degrees in Meteorology and
Environmental Science (physics focus) from Rutgers
University. Her research interests include cloud microphysics,
aerosol-cloud-American Meteorological Society, cited 2020:
Precipitation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation-American Meteorological
Society, cited 2019: Climate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Climate.]"
ATMO 200 • 11
class="glossaryLink">climate
interactions,
meteorology, drought, and satellite meteorology.
aviation
Dr. Griswold had the initial idea for the OER textbook and
applied for the Outreach College grant that made the project
possible. She teaches undergraduate non-major survey courses
covering a variety of American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather and American Meteorological
Society, cited 2019: Climate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Climate.]"
class="glossaryLink">climate topics and graduate level
specialty courses in satellite meteorology and data analysis. One
of her main goals is to improve the diversity of the atmospheric
sciences community by encouraging underrepresented groups
(e.g., Native Hawaiians and Pacific Islanders) and women to
pursue careers in atmospheric science.
12 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Britt Seifert
Britt Seifert created figures, checked copyright content, and
migrated content into Pressbooks. She is a current undergraduate
student in the Department of Atmospheric Sciences at the
University of Hawai‘i at Mānoa.
ATMO 200 • 13
May Izumi
The text was copyedited by May Izumi, a publications editor for
the School of Ocean and Earth Science and Technology at the
University of Hawai‘i at Mānoa.
14 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Billy Meinke-Lau
Billy Meinke-Lau works for the Outreach College at the
University of Hawai‘i at Mānoa, supporting faculty to develop
Open Educational Resources (OER). He previously worked with
the Distance Course Design and Consulting (DCDC) Group in
the College of Education at the University of Hawai‘i at Mānoa,
and for Creative Commons.
None of this would have been possible without the help and
guidance of Billy Meinke who served as project manager. His
passion for Open Educational Resources is obvious, and guides
his American Meteorological Society, cited 2019: Work.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Work.]"
class="glossaryLink">work and actions. Thank you Billy, your
American Meteorological Society, cited 2019: Energy. Glossary
ATMO 200 • 15
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy and devotion to
OER is inspiring.
Acknowledgements
This Open Educational Resource textbook has been adapted
from Roland Stull’s OER textbook entitled “Practical
Meteorology: An Algebra-based Survey of Atmospheric
Science.” It has been simplified and the content was reduced to
meet the needs of the ATMO 200 course at the University of
Hawai‘i.
Chapters and section structures were adapted from the above
existing OER textbook. Without this foundational text, a lot
more work would have been required to complete this project. A
huge thank you to Roland Stull.
All shared content should include the following attribution
statement:
—
This work is licensed under a Creative Commons Attribution
4.0 International License. Atmospheric Science: A Companion
16
ATMO 200 • 17
Text for ATMO 200 by the University of Hawai’i at Mānoa
Atmospheric Sciences Department.
—
Front Cover Photo
Rainbow and Halema’uma’u eruption (CC BY-SA 4.0)
Special Thanks
Roland Stull — University of British Columbia (link)
Bill Chismar — University of Hawai’i at Mānoa, Dean of the
Outreach College
Open Educational Resources
This text is provided to you as an Open Educational Resource
(OER) which you can access online. It is designed to give you a
comprehensive introduction to atmospheric science at no cost.
Chapter 1: Atmospheric Basics
ALISON NUGENT AND DAVID DECOU
Learning Objectives
By the end of this chapter, you should be able to:
1.
Convert between temperature units of
Fahrenheit, Celsius, and Kelvin
2.
Use mathematical formulas to define
atmospheric temperature, pressure, and
density
3.
Compute pressure and density changes
with altitude
4.
Describe the vertical structure of
Earth’s atmosphere
19
20 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
5.
Define and apply the ideal gas law
6.
Describe American Meteorological
Society, cited 2019: Hydrostatic balance.
Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/
wiki/Hydrostatic_balance.]"
class="glossaryLink">hydrostatic
balance
7.
Discuss the difference between
American Meteorological Society, cited
2019: Weather. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Weather.]"
class="glossaryLink">weather and
American Meteorological Society, cited
2019: Climate. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Climate.]" class="glossaryLink">climate
8.
Note the location of terminology,
coordinate systems, and units for future
reference
Introduction
We often take Earth’s atmosphere for granted. When we enjoy
a sunset, go to the beach, or hike a trail, we don’t necessarily
think of the vast volume of air around us that allows us to do
ATMO 200 • 21
these things. The atmosphere brings us the oxygen that fills our
lungs; it brings us the beautiful blues of the sky above and the
verdant greenery below. It is responsible for the existence of
our oceans, lakes, and rivers, and without it we would not have
beaches to enjoy. The fluffy, white cumulus clouds you see on a
summer day are a result of both large- and small-scale motions
in the atmosphere. The same is true of the hazy stratus, wispy
cirrus, or towering cumulonimbus. The atmosphere is like a
massive blanket that surrounds, sustains, protects, and warms us.
Without it, no life would exist on Earth. In fact, the atmosphere
is the very reason almost anything exists on Earth at all; our
planet would be a dull, waterless, lifeless rock without it. Nighttime without an atmosphere would be unimaginably cold, and
daytime temperatures would soar above water’s boiling point.
Nothing would separate Earth and the blistering sunlight but the
empty vacuum of space.
View from outer space of the sun rising over Earth, illuminating the
atmosphere in a ring of blue. NASA (Public Domain).
22 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Although the atmosphere was previously described as “vast”,
in actuality it is relatively thin. If the Earth were shrunk down
to the size of a basketball, the atmosphere would be roughly
the thickness of a plastic sheet stretched across the ball. In
the image above, the atmosphere can be seen as the thin veil
of bluish white mist above our planet’s surface. Although we
can travel thousands of kilometers horizontally along Earth’s
surface, traveling vertically would be difficult — the air is too
thin to breathe only a few kilometers above Earth’s surface.
View of the moon from space over Earth’s blue atmosphere (Public
Domain).
The goal of this chapter is to orient those who are new to
atmospheric science with the basics as described previously in
the Chapter 1 Learning Objectives.
ATMO 200 • 23
Overview of Earth’s Atmosphere
Everything that happens on Earth is caused in some way by
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation from the sun, which is an
average-sized star located near the edge of the Milky Way
galaxy. It provides radiant American Meteorological Society,
cited 2019: Energy. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy
(also
called
American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation or solar
insolation) by converting hydrogen into helium near its core,
which provides most of the Earth’s warmth. Here is a classic
song about the Sun heating Earth.
The Earth only receives a tiny portion of the sun’s total
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy output. It is this
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation that drives the atmosphere’s
wind and American Meteorological Society, cited 2019:
Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
24 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">weather patterns on the Earth’s surface.
It is because of this solar input that the Earth can maintain
an overall global average surface temperature of approximately
15°C (59°F).
Composition of the Atmosphere
Pie chart showing the gases that make up Earth’s atmosphere (CC
BY-SA 3.0).
Earth’s atmosphere is primarily composed of nitrogen (N) and
oxygen (O), with smaller quantities of other gases, as shown
in the figure above. Nitrogen makes up around 78%, oxygen
makes up around 21%, and Argon almost 1% of the total dry
air volume in the atmosphere. Other gases (such as water vapor,
the gaseous form of H2O) take up varying amounts depending
on the location and atmospheric conditions. There is a balance
of input and output of atmospheric gases at the Earth’s surface
both from life and surface processes. For example, when we
ATMO 200 • 25
breathe, the human body takes in oxygen (O2) and releases
carbon dioxide (CO2). When plants photosynthesize, they take
in carbon dioxide and release oxygen. When water (H2O)
evaporates from the ocean, additional water vapor (H2O) is
added to the atmosphere. When the Kilauea volcano degasses,
sulfur dioxide (SO2) is added to the atmosphere.
Water Vapor
Water vapor concentrations vary greatly from location to
location, and from time to time. In warm, moist, tropical regions,
water vapor can make up nearly 4% of the atmospheric
composition, while in polar regions it can be just a tiny fraction
of a percent. Water vapor cannot be seen in its gaseous form,
but you may see it as it condenses into water droplets on the
side of a cool glass of water on a hot day or into water droplets
that make up clouds. The process of water vapor changing phase
to liquid is called condensation. When liquid water becomes a
gaseous vapor, it is known as evaporation. When water becomes
ice or ice becomes water, these processes are known as freezing
and melting, respectively. When ice transitions directly to water
vapor, the process is known as sublimation. The transition from
water vapor to ice is known as vapor deposition. These terms
will all be used frequently in subsequent chapters.
Water vapor is one of the most important gases in our
atmosphere, because when it changes phase from vapor to liquid
or ice, it releases enormous amounts of heat, called American
Meteorological Society, cited 2019: Latent heat. Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Latent_heat.]"
26 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">latent heat. American Meteorological
Society, cited 2019: Latent heat. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Latent_heat.]" class="glossaryLink">Latent heat is a major
source of American Meteorological Society, cited 2019: Energy.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy in the atmosphere, particularly for
tropical storms and other types of convection. In addition, water
vapor is an important greenhouse gas in the atmosphere,
meaning it absorbs and re-releases a portion of the Earth’s
outgoing American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation, which allows our planet to
remain warm.
If you don’t understand convection, American Meteorological
Society, cited 2019: Latent heat. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Latent_heat.]" class="glossaryLink">latent heat, and American
Meteorological Society, cited 2019: Greenhouse gases. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Greenhouse_gases.]"
class="glossaryLink">greenhouse
gases yet, don’t worry, all will be discussed in later chapters. The
point is that despite its small fractional percentage, water vapor
is arguably the most important gas in the atmosphere.
ATMO 200 • 27
Hawaiian Focus Box
The air composition of Hawaiʻi occasionally faces
a unique threat due to the emissions from Kīlauea, a
volcano located along the southern shore of
Hawaiʻi’s Big Island. Kīlauea has been continuously
erupting since 1983 and is the most active of the
volcanoes that make up the Big Island. In the first
few years after 1983, Kīlauea emitted up to 30,000
tons of sulfur dioxide (SO2) a day, but that number
has stabilized recently to around 5,000 tons per day.
At present day with the 2018 Kilauea eruptions
currently occurring, SO2 emissions are again
elevated. These emissions are known as volcanic
smog, or more commonly, “vog”. The sulfate
particles in vog are very small, so they can
effectively infiltrate human airways. In environments
with high relative humidity, such as inside the human
body, these particles will hydrolyze and expand,
which irritates lungs and obstructs airways. Vog has
been linked to respiratory disease, sinusitus, and
even lung cancer. Therefore, Hawaiʻi’s residents
need to take precautions.
28 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Water vapor and other gases escaping from the Kīlauea
volcano at the Halemaʻumaʻu vent (CC BY-SA 4.0).
Vertical Structure of the Atmosphere
Although the atmosphere extends vertically for hundreds of
kilometers, almost 99% of it is within approximately 30 km of
the Earth’s surface. Air molecules are pulled toward Earth by the
gravitational force, which pulls downward on the atmosphere.
This causes the air molecules closer to Earth’s surface to be more
tightly compressed, meaning that there are more air molecules
together in a given volume (a higher density). The greater the
number of air molecules that exist above a certain altitude, the
greater the effect of this compression, because the molecules are
all being pulled downward together. Just as gravity has an effect
on the weight of different objects (weight is the force that acts
on an object due to gravity), it also gives weight to air.
ATMO 200 • 29
Mass is the measure of how much matter exists in a given object
or space. Mass is occasionally confused with weight. While
weight increases with more mass, mass would have no weight
at all without the pull of gravity. Similarly, an object’s weight
depends on the gravitational force: an object weighs much less
on the moon than it does on Earth. Mass is typically given in
grams (g) or kilograms (kg).
Note: You may want to review the International
System (IS) of Units. The base units we will use
include meters (m), kilograms (kg), seconds (s), and
Kelvin (K). We’ll also use the prefixes “kilo” (1000),
“hecto” (100), and “micro” (10-6) along with some
derived units like hertz, Newtons, pascals, joules, and
watts.
Air Density
Air density is determined by the amount of mass (m) that exists
in a given space, or volume (V). If you fit a lot of mass into a tiny
volume, you will have higher density. If you have only a small
amount of mass spread out over a large volume, the density will
be lower.
30 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Putting these ideas together, air density is highest near the
surface of Earth and decreases with height, because there are
more molecules held tightly together at the surface by gravity
than there are above. The standard unit for density is kg·m-3.
Pro Tip: Density is often denoted by the Greek
lowercase letter ρ (rho). Be careful not to confuse this
with p or P, which are both used to denote pressure.
When writing on paper, it may be useful to write ρ
with a curly tail like a backwards q and to write p with
a straight tail in order to differentiate the two.
Density (ρ, kg·m-3) vs. height (z, km) in the atmosphere
(CC BY-NC-SA 4.0).
The image above shows the distribution of density (ρ) with
ATMO 200 • 31
altitude. Atmospheric surface density is typically around 1.2
kg·m-3. We call this value ρ0 because it is the initial value at
the surface. The exponential equation below approximates the
distribution of density with height
where ρ (kg m-3) is density, ρ0 (kg m-3) is the density at the
Earth’s surface, z (m) is altitude, T (K) is temperature (assumed
to be constant through the atmosphere), Rd = 287.053
J·K-1·kg-1 (gas constant for dry air), and g is the acceleration
due to gravity (m·s–2).
Note: Remember that the equations you see in this
text are good approximations. They all involve
assumptions of some kind. In the case of the
exponential distribution of density with altitude, the
primary issue is the assumption that temperature is
constant throughout the atmosphere. This will become
clear later.
“Scale height” is denoted by H and represents an e-folding
distance for the drop off of density in the atmosphere. Typically
the value of H is around 8000 m. This means that density at 8
km is approximately 1/e (e=2.71828) of the value at the surface.
A back of the envelope calculation gives the density of the
atmosphere at 8 km as 1.2 kg·m-3 divided by 3 and is
32 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
approximately equal to 0.4 kg·m-3. Checking the image above, it
is clear that this provides a relatively good estimate for density.
Air Pressure
You will often hear meteorologists on TV discuss the air
pressure, or you might see H’s and L’s on a map displayed
on the American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">Weather Channel, denoting high and low
pressure areas. Because air molecules are in constant motion,
they collide with one another and other objects up to several
billions of times a second. Each time an air molecule collides
with an object, it exerts a tiny amount of force. Air pressure
refers to the total force that air exerts against a given area of an
object.
A Newton (N) is the unit for force and m2 is the unit for area,
in the International System of Units (SI). Therefore, the standard
unit for pressure is in Newtons per square meter — or Pascals
(Pa), which are defined as 1 N·m-2.
ATMO 200 • 33
Pro Tip: You will generally find air pressure
expressed in units of millibars or (mb) or hectopascals
(hPa) on American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather maps. These two units
are equivalent to one another. In the Practical
Meteorology: An Algebra-based Survey of
Atmospheric Science textbook by Roland Stull off of
which this OER is based, kPa are often used. In the
aviation field, Inches of mercury (inHg) are also
commonly used. At sea level, the global average for
atmospheric pressure is:
101.325 kPa = 1013.25 mb = 1013.25 hPa = 29.92
in. Hg. = 1 atm (atmosphere) = 101325 Pa.
In future calculations, you will usually need to
express pressure in Pascals (Pa) for your units to
cancel out. Always be mindful of the units you are
given and be sure you are able to convert from one
type of unit to another.
You can think of the surface air pressure as the total weight of a
column of air molecules extending from the surface to the top of
the atmosphere.
Because there are more air molecules at the surface of the Earth
and less above, air pressure is maximized at the surface and
decreases with height nearly exponentially, analogous to air
density.
34 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Height (z, km) vs. pressure (P, kPa) in the atmosphere
(CC BY-NC-SA 4.0).
The image above shows the distribution of pressure (P) with
altitude. Atmospheric surface pressure is typically around 1013
hPa. We call this value P0 because it is the initial value at
the surface. The exponential equation below approximates the
distribution of pressure with height
where P (Pa) is pressure, P0(Pa) is the pressure at the Earth’s
surface, z (m) is altitude, T (K) is temperature (assumed to be
constant through the atmosphere), Rd = 287.053 J·K-1·kg-1 (gas
constant for dry air), and g is the acceleration due to gravity
(m·s–2).
You can get a sense of atmospheric pressure if you’ve ever dived
ATMO 200 • 35
more than a few feet underwater at the pool or beach. As you
dive deeper, the weight of the water above you increases, and
you feel increased discomfort or pressure on your head. This is
why divers require special equipment to reach greater depths in
the ocean. If you think of yourself at the bottom of an ocean of
air, you can imagine why air pressure is highest down here at the
surface. If you’ve ever flown on an airplane, you can also feel
the changes in atmospheric pressure in your ears as you ascend
and descend. Still, while pressure decreases at high elevations in
an airplane, cabins are pressurized to maintain similar pressure
to Earth’s surface so you are not experiencing the full pressure
drop with altitude.
Air Temperature
It is commonly thought that air temperature is a measure of how
cold or hot an object is. We instinctively know that the higher
an object’s temperature, the hotter it will be, and the lower its
temperature, the colder it will be. But what does temperature
really measure? The answer lies in the motion of the molecules.
Air molecules are in constant motion, colliding with objects
and one another. As we increase the temperature of a volume
of air, the air molecules speed up, and collisions become more
frequent. As we decrease the air temperature, the molecules slow
down, and collisions occur less frequently. What is going on
here? Simply put, air temperature is a relative measure of the
average American Meteorological Society, cited 2019: Kinetic
energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Kinetic_energy.]"
36 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">kinetic energy (kinetic meaning it relates
to motion) of the molecules of a system.
Because there are more air molecules close to the Earth’s
surface, density and air pressure are maximized at the surface
and decrease with height. Based on this, do you think air
temperature will decrease or increase as you move further away
from the surface?
Within the lowest 10-12 km of the atmosphere, temperature
tends to decrease with height, primarily because the Earth’s
surface is warmed by sunlight, which then warms the layer of
air directly above it, which warms the air above that, and so on.
While this is true in the lowest surface layer of the atmosphere,
this is not true throughout the rest of the atmosphere. The
vertical temperature profile is more complicated than the vertical
profile of pressure or density. Temperature is different for each
of the different layers of the atmosphere, which will be discussed
later.
Temperature Scales
Surface air temperature is commonly given in degrees
Fahrenheit (°F) in the United States. If you were born and
raised in the US, this is the temperature scale that you probably
use in your daily life, and what you’ll see given when you
look at the daily American Meteorological Society, cited 2019:
Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather. You’ll know that 32°F is the
ATMO 200 • 37
temperature at which water freezes, and 212°F is the temperature
at which water boils.
Degrees Celsius (°C) is also commonly used, especially
internationally. Celsius is a convenient scale to use because
water freezes at 0°C and boils at 100°C. A difference in 1°C is
larger than a difference of 1°F by about 1.8 times. To convert
between the two, use the following equations.
The Kelvin scale (K) is almost always used as a temperature unit
in scientific equations and is convenient in that it contains no
negative numbers. The Kelvin scale begins at 0 K, or absolute
zero, where atoms and molecules would theoretically be
thermally motionless. The Kelvin scale is also sometimes known
as the absolute temperature scale. The lowest possible
temperature is 0 K but it does not occur naturally. The coolest
naturally existing place known in the universe is the Boomerang
Nebula, located in the Centaurus constellation about 5,000 lightyears away from Earth. The temperature is measured at 1 K, only
1°C above absolute zero. A difference in 1 K is the same as a
difference of 1°C, so a conversion is linear and simple.
Based on this, absolute zero is -273.15 °C. Keep in mind degrees
38 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Celsius (°C) and degrees Fahrenheit (°F) always have the degree
° symbol in American Meteorological Society, cited 2020: Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front, but Kelvin (K) never uses this
symbol. Typical values of temperature on Earth’s surface on the
Kelvin scale are values around 260-310 K.
Within atmospheric sciences, the Kelvin and Celsius scale are
used. This chapter is one of the only times you’ll see a discussion
of Fahrenheit within the course.
Equation of State — Ideal Gas Law
Air pressure is caused by the collisions of rapidly, randomly
moving air molecules, so you might expect pressure to increase
when there are more molecules in one place (higher density, ρ)
and when they are moving faster (higher temperature, T). The
relationship between pressure, density, and temperature is called
the American Meteorological Society, cited 2019: Equation
of state. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Equation_of_state.]"
class="glossaryLink">Equation of State.
The gases in the atmosphere have a simple American
Meteorological Society, cited 2019: Equation of state. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Equation_of_state.]" class="glossaryLink">equation of
state called the Ideal Gas Law.
ATMO 200 • 39
For dry air (no water vapor present), the ideal gas law is
The ideal gas law assumes that atmospheric gases act in an ideal
manner, meaning that there are few intermolecular forces, and
that the size of the molecules is small compared to the space
between them. In almost all cases, gases in the atmosphere can
be assumed to be ideal gases. Again, this is another example
of a simplifying assumption we use to be able to approximate
relationships in the atmosphere.
Air pressure differences associated with air temperature in a column (CC
BY-SA 4.0). Description below.
The above figure illustrates the effect that air temperature has on
its density and pressure. The two air columns above Cities 1 and
2 in (a) have the same temperature, contain the same amount of
mass, and have the same volume, so their density is the same.
The pressure exerted on the surface is the same in both cities
because the pressure at the surface is related to the number of air
molecules above.
In (b) the air temperature above City 1 is lower than the air
40 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
temperature above City 2. Because of this the air molecules in
the column above City 1 are moving more slowly, and take
up a smaller volume. Likewise, in City 2 the air molecules are
moving more quickly and the volume in the column is larger. It
only takes a smaller cold air volume to apply the same pressure
on the surface as a larger warm air volume. The surface
pressures are the same but the temperatures are not.
The surface pressure may be the same in (b) but the pressure
aloft is not, which results in air movement that can be seen in
(c). At the same altitude above Cities 1 and 2 in (c), there is
more air above the same level in City 2 than in 1. Locally this
creates a higher pressure aloft over the warmer City 2. Because
of the difference in pressure aloft, airflow is created that moves
from the higher pressure toward the lower pressure. This airflow
caused by a localized difference in air pressure is what we call
wind.
Note: The force that arises due to a difference in air
pressure is called the American Meteorological
Society, cited 2020: Pressure Gradient Force.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Pressuregradient_force.]" class="glossaryLink">pressure
gradient force (PGF). The word “gradient” here
refers to a change over a distance. A American
Meteorological Society, cited 2020: Pressure Gradient
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Pressuregradient_force.]" class="glossaryLink">pressure
gradient force is the force that drives the wind. This
ATMO 200 • 41
concept will be further explored in Chapter 10.
The movement of air from City 2 to City 1 creates a falling
surface pressure in City 2 and a rising surface pressure in City 1.
Hydrostatic Balance
We’ve learned that atmospheric pressure decreases with height.
We’ve also learned that air moves from areas of high pressure
to areas of low pressure due to the American Meteorological
Society, cited 2020: Pressure Gradient Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Pressure-gradient_force.]" class="glossaryLink">pressure
gradient force. Knowing these two things, you might think that
the air in the atmosphere would escape into space, because there
is high pressure at the surface and low pressure aloft. Because
it does not, there must be a downward force that balances the
upward vertical American Meteorological Society, cited 2020:
Pressure Gradient Force. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Pressuregradient_force.]" class="glossaryLink">pressure gradient force.
This downward force is a force that should be very familiar to
you: gravity.
In the atmosphere, when the vertical American Meteorological
Society, cited 2020: Pressure Gradient Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Pressure-gradient_force.]" class="glossaryLink">pressure
gradient force is balanced by gravity, this is known as American
42 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorological Society, cited 2019: Hydrostatic balance.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Hydrostatic_balance.]"
class="glossaryLink">hydrostatic balance. The word hydro
means water or fluid, and static means stationary, so the name
can be interpreted as a stationary fluid balance. This balance
holds true for most situations in the atmosphere. The hydrostatic
equation is given by
where g = -9.8 m·s-2 is the acceleration due to gravity.
The negative sign here is due to the fact that pressure is
decreasing as height increases so the left-hand side will be
negative.
If you plan to use the above equation to calculate changes in
altitude (Δz) with changes in pressure (ΔP) or vise versa, note
that the above equation applies best over small changes. If the
change in pressure or altitude is large, the exponential equation
for P(z) defined above is best.
Hypsometric Equation
You have learned that air pressure decreases with height, but
sometimes we want to know the distance or thickness between
two different pressure levels. The atmospheric thickness varies
depending on the average temperature in the layer. Warmer air is
spaced out more, so a warmer layer of air will be thicker, while
ATMO 200 • 43
a cooler layer of air will be thinner. By knowing the average
temperature of the layer, and the top and bottom pressure levels,
you can calculate the thickness of the atmospheric layer. The
American Meteorological Society, cited 2019: Hypsometric
equation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Hypsometric_equation.]"
class="glossaryLink">hypsometric equation allows you to
calculate how pressure varies with height in an atmosphere
regardless of the temperature profile. It is the result of
combining the ideal gas law with the hydrostatic equation. The
American Meteorological Society, cited 2019: Hypsometric
equation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Hypsometric_equation.]"
class="glossaryLink">hypsometric equation allows you
to calculate the thickness ( z2 – z1 ) between two pressure levels,
P2 and P1. The z2 and z1 values are the heights at pressure levels
P2 and P1, respectively.
In the above equation, the average temperature is shown. If the
atmosphere is very moist, you may wish to use the average
American Meteorological Society, cited 2019: Virtual
temperature. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Virtual_temperature.]"
44 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">virtual temperature between the two
heights z2 and z1 instead which includes the effects of water
vapor.
Tv is known as the American Meteorological Society, cited
2019: Virtual temperature. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Virtual_temperature.]"
class="glossaryLink">virtual
temperature, defined as:
where the mixing ratio (r) is the mass of water vapor per mass of
dry air and uses units of kilograms of water vapor per kilograms
of dry air (kg·kg-1). You will learn that many times American
Meteorological Society, cited 2019: Virtual temperature.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Virtual_temperature.]"
class="glossaryLink">virtual temperature can be used inside of
equations in place of temperature if the effect of water vapor
needs to be included.
Layers of the Atmosphere
In the previous sections, we talked about how both air pressure
and density decrease with height, because most of the
atmosphere is held close to the Earth’s surface. This change in
pressure and density occurs quickly at first, but slows down at
higher altitudes. Unfortunately the change in temperature with
altitude is not nearly as straightforward. When we look at a
ATMO 200 • 45
vertical profile of the atmosphere, we see that it can be divided
into different layers in a number of ways. We can define layers
by how the air temperature varies with height, by the gas
composition, and even by the electrical properties of each layer.
46 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
The vertical temperature structure of Earth’s atmosphere (Public
Domain) in kilometers (left axis) and miles (right axis). The layers are
labeled as “spheres” and the boundaries between the layers are labeled
as “pauses”. For example the troposphere is the lowest layer and the
tropopause is the boundary between the troposphere and the
stratosphere. The yellow line shows temperature in °C, and the cloud
indicates where most of the weather occurs.
As can be seen in the above figure, the air temperature decreases
with height up to the tropopause, 11 km in this example. This
ATMO 200 • 47
occurs because American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation from the sun warms the Earth’s
surface, and the surface warms the air just above it. This will be
discussed in further detail in later chapters.
Troposphere
The layer of the atmosphere that we are most familiar with
is the troposphere, which extends from the surface to around
11 km. All of the every day American Meteorological Society,
cited 2019: Weather. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather that we experience on Earth
happens within the troposphere, which is characterized by
frequent rising and sinking vertical air motions. The troposphere
gets its name from the root word “tropo” in Greek, which means
turning.
At the top of the troposphere, the air temperature stops
decreasing with height in a region known as the tropopause,
which separates the troposphere and the stratosphere above. The
height of the tropopause varies depending on the season and
location. In warmer areas near the equator, the tropopause tends
to be higher (around 17 km), while in colder polar regions the
tropopause is lower (around 9 km) because warm layers of air
are thicker than layers of cold air. For the same reason, the
tropopause is found at higher elevations in the summer, and at
48 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
lower elevations in the winter. Aircraft fly at the tropopause
height.
Atmospheric Boundary Layer
The American Meteorological Society, cited 2019:
Atmospheric boundary layer. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Atmospheric_boundary_layer.]"
class="glossaryLink">atmospheric boundary layer (ABL) is
located within the lowest 0.3 to 3 km of the troposphere and is
affected in several ways due to its close proximity with the
Earth’s surface. Airflow closest to the surface slows down due
to the effect of friction, which can be amplified due to the
type of terrain or amount of vegetation. Because of this, the
boundary layer experiences the greatest amount of turbulence in
the atmosphere. In addition, the boundary layer is warmed by the
Earth’s surface during the daytime, so it is the layer that is most
affected by the diurnal (day-night) heating cycle.
ATMO 200 • 49
The change in temperature with altitude in the
troposphere which is the lowest layer of Earth’s
atmosphere. The turbulent boundary layer near the
surface is also seen (shaded tan), along with the standard
atmospheric lapse rate (-6.5 K·km-1), and near-surface
day (red) / night (blue) temperature differences (CC
BY-NC-SA 4.0).
Stratosphere
In the stratosphere, the air temperature increases with height,
causing a temperature inversion. This inversion layer tends to
keep rising and sinking tropospheric air from mixing with
stratospheric air. It also prevents rising and sinking from
happening in the stratosphere itself. Because of this, it is called
a stratified layer. The root word “strato” means layered, or
spreading out, which is a good way to describe the many layers
of the stratosphere. The temperature of the stratosphere increases
with height because of the presence of a gas called ozone (O3),
50 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
which heats the air through the absorption of ultraviolet (UV)
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation from the sun.
At around 50 km, where the stratosphere is warmest (due to
most of the UV American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation absorption occurring in the
uppermost parts of the layer). The boundary known as the
stratopause separates the stratosphere below from the
mesosphere above.
Mesosphere
In the mesosphere, the air temperature once again decreases with
height and the air is extremely thin with a low density. The
average atmospheric pressure in this layer is about 1 mb, which
means that about 99.9% of all air molecules are located below
this level and only about one thousandth of the atmospheric
mass is located above. You would not survive very long in
the mesosphere, however, due to the extreme thinness of the
air, freezing temperatures, and direct exposure to ultraviolet
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation. In addition, your blood would
begin to boil at normal body temperatures if it were directly
ATMO 200 • 51
exposed to the low air pressure. The reason that temperatures
decrease with height in the mesosphere is due partially to the
fact that there is not a lot of ozone to absorb UV American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation. Because of
this, the air molecules in the mesosphere lose more American
Meteorological Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy than they absorb.
At about 85 km near the mesopause, the atmosphere is at its
coldest at about -90°C. The mesopause separates the mesosphere
from the thermosphere above.
Thermosphere
In the thermosphere, the air density is extremely low so even a
small amount of American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation absorption can lead to a large
increase in temperature. Molecules can travel for entire
kilometers without colliding into another molecule. This is the
layer where auroras occur as a result of interactions between
charged solar particles and air molecules.
At the top of the thermosphere, at above 500 km above the
Earth’s surface, molecules can actually escape the gravitational
52 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
pull of the Earth. This region is known as the exosphere, and
represents the upper limit of the atmosphere.
ATMO 200 • 53
This figure illustrates the relative heights of different phenomena in the
various vertical layers of the atmosphere. The Kármán Line at 62 miles
(100 km) designates the boundary between Earth’s atmosphere and outer
space. Image by NOAA Satellites (Public Domain).
54 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
The previous figure shows the levels in Earth’s atmosphere and
the phenomena and activities that occur there. For example,
commercial jet airplanes fly at the base of the stratosphere,
while the International Space Station is positioned in the
thermosphere.
Chapter 1 contained a vast array of topics, from defining
temperature and pressure, to describing atmospheric vertical
structure and components. As a reminder, these were our
learning goals:
1. Convert between temperature units of Fahrenheit,
Celsius, and Kelvin
2. Use mathematical formulas to define atmospheric
temperature, pressure, and density
3. Compute pressure and density changes with altitude
4. Describe the vertical structure of Earth’s atmosphere
5. Define and apply the ideal gas law
6. Describe American Meteorological Society, cited
2019: Hydrostatic balance. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Hydrostatic_balance.]"
class="glossaryLink">hydrostatic balance
7. Discuss the difference between American
Meteorological Society, cited 2019: Weather.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
ATMO 200 • 55
class="glossaryLink">weather and American
Meteorological Society, cited 2019: Climate.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Climate.]"
class="glossaryLink">climate
8. Note the location of terminology, coordinate systems,
and units for future reference
The next section will discuss American Meteorological Society,
cited 2019: Weather. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather vs. American Meteorological
Society, cited 2019: Climate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Climate.]"
class="glossaryLink">climate and various terms and coordinate
systems that are useful to review.
Weather and Climate: What’s the Difference?
Occasionally the terms “American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather” and “American Meteorological
Society, cited 2019: Climate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Climate.]"
class="glossaryLink">climate” get confused with one another.
American Meteorological Society, cited 2019: Climate. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Climate.]" class="glossaryLink">Climate change has been
56 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
a hot topic in recent decades. Knowing this, you might conclude
that when the American Meteorological Society, cited 2019:
Climate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Climate.]"
class="glossaryLink">climate changes — it must be a pretty
big deal, however, the American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather is in a constant state of change. If
it weren’t, we would have little need to check the Internet or
local American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather station to see what the sky is
going to look like tomorrow. Nobody bats an eye when the
American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather is different from one day to the
next. What do we mean exactly by the terms American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather and American
Meteorological Society, cited 2019: Climate. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Climate.]" class="glossaryLink">climate?
American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
ATMO 200 • 57
class="glossaryLink">Weather refers to the present condition
of the atmosphere at any given time and place. This includes the
following elements.
Air Temperature — how hot or cold the air is.
Air Pressure — the force the air exerts on the surface.
Humidity — the amount of water vapor present in the air.
Clouds — masses of water droplets and/or ice crystals that
obscure parts of the sky.
American Meteorological Society, cited 2020: Precipitation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">Precipitation — water (solid or liquid)
that falls from clouds and reaches the ground
Visibility — the maximum horizontal distance that can be seen.
This can be affected by the presence of fog or American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation.
Wind — the horizontal flow of air, caused by local differences
in air pressure.
American Meteorological Society, cited 2019: Climate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Climate.]"
class="glossaryLink">Climate represents the average range
of American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather events in a region over a long
58 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
period of time including American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather extremes such as heat waves or
cold spells, as well as the frequency of these events.
One way to think of it is American Meteorological Society,
cited 2019: Weather. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather influences the kind of clothing
we might wear that day, e.g., Will I need a raincoat? Is it too
warm to wear long sleeves? Is it too cool to wear shorts?
American Meteorological Society, cited 2019: Climate. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Climate.]" class="glossaryLink">Climate, on the other
hand, influences what clothing we buy. For example, in Hawaiʻi,
it isn’t likely that you would need to buy a heavy winter jacket.
Chapter 1 Reference Guide: Coordinate Systems,
Units, Terminology
This reference guide covers some topics that are outlined in
Roland Stull’s Practical Meteorology: An Algebra-based Survey
of Atmospheric Science Chapter 1, which may be useful to you
going forward. These topics include the following.
Meteorological Conventions:
This section describes standard meteorological conventions that
are necessary to keep in mind when solving problems in this
ATMO 200 • 59
course. It includes different coordinate systems, including
Cartesian coordinates, polar coordinates, and spherical
coordinates. You are likely already well acquainted with
Cartesian coordinates, but polar and spherical coordinates may
be new to you. The standard way of describing and plotting wind
direction is also given here.
Cartesian coordinates: x, y, and z and velocity: u, v, and w.
Polar coordinates: direction and magnitude.
Wind in Cartesian coordinates: (U, V)
Wind in Polar coordinates: (α, M)
Algebraically, wind is given a magnitude (wind speed) and a
direction. Wind direction is split up into different components.
When only horizontal wind motion is taken into account, we use
U and V, and vertical air motions are denoted by W.
U -> Wind in the x-direction
V -> Wind in the y-direction
W -> Wind in the z-direction (vertical air motion)
Because of this, wind can be plotted using polar coordinates,
wind direction (dir) and wind magnitude (spd).
Wind directions are given by angle, with 0° to the north, and
degrees increasing clockwise. Winds are described using the
direction from which they come. A westerly wind is a wind
from the west. Hawaiʻi is often impacted by the easterly or
northeasterly American Meteorological Society, cited 2020:
Trade Winds. Glossary of Meteorology. [Available online at
60 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://glossary.ametsoc.org/wiki/Trade_winds.]"
class="glossaryLink">trade winds, which come from the east
and northeast.
Sometimes, cylindrical coordinates (M, α, W) are used which
are similar to polar coordinates in that magnitude and direction
of wind velocity are used, but also include the vertical motion
component W.
The Stull text uses the words “ordinate” and “abscissa”.
Ordinate is simply the vertical axis (typically the y-axis) and
abscissa is the horizontal axis (typically the x-axis). Independent
variables are usually plotted on the x-axis, with dependent
variables plotted on the y-axis.
Earth Frameworks Reviewed:
The Earth is not a sphere, but it is pretty close. The distance
between the center of the Earth and the north and south poles
differs by about 20 km, so the Earth is referred to as an oblate
spheroid.
Cartography:
Meridians, which are north-south lines on a globe, are given by
ATMO 200 • 61
degrees longitude. Think of the distance between the north and
south poles as long.
The prime meridian lies at 0° longitude, and passes through
Greenwich, Great Britain.
East of here (defined as the Eastern Hemisphere, 0 – 180 °E),
longitude is positive.
West of the prime meridian (Western Hemisphere, 0 – 180 °W),
longitude is negative.
The Earth rotates counterclockwise about its axis.
Parallels, which are east-west lines on a globe, are given by
degrees latitude. A good way to remember this is “Lat” rhymes
with “flat” — just like the east-west horizontal (flat) lines of
latitude.
The Equator is 0° latitude, with latitudes north of the Equator as
positive (Northern Hemisphere: 0 – 90°N), and latitudes south
of the Equator are negative (Southern Hemisphere: 0 – -90°S).
A helpful approximation between degrees of latitude and
distance is that each degree of latitude is approximately 111 km,
or about 60 nautical miles.
Chapter 1: Questions to Consider
1.
Drag and drop the correct labels to the
appropriate layers of the atmosphere.
62 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=5
2.
Mauna Kea on Hawaiʻi Island is
13,803 feet tall at the summit. If the
pressure at sea level is 1015 hPa and the
scale height is 8 km, what is the pressure
at the summit? (Hint: don’t forget to
check the units!)
3.
The pressure at the ground floor of a
high-rise building is 1012 hPa. On the
roof the pressure is 1007 hPa. If you
assume that the air density is 1.2 kg·m-3
and acceleration due to gravity is -9.8
m·s-2, approximately how tall is the
building?
4.
In this chapter you learned about two
different equations that describe the
change in air pressure (P) with altitude
(z):
When is the exponential equation
ATMO 200 • 63
preferable? When is the hydrostatic equation
preferable? In questions two and three above,
which equation did you use, and would your
answers change if you used the other instead?
Think about the assumptions that go into each
equation and which variables are assumed to
remain constant while pressure varies.
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/atmo/?p=5
Chapter 2: Solar and Infrared
Radiation
ALISON NUGENT AND SHINTARO RUSSELL
Learning Objectives
By the end of this chapter, you should be able to:
1.
Define black body American
Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Radiation.]"
class="glossaryLink">radiation and
Planck’s Law
64
ATMO 200 • 65
2.
Apply American Meteorological
Society, cited 2019: Wien’s law. Glossary
of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Wien%27s_law.]"
class="glossaryLink">Wien’s law to
compute the maximum emission
wavelength
3.
Use Stefan-Boltzmann’s law to
compute radiative emittance
4.
Describe Earth’s surface American
Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Radiation.]"
class="glossaryLink">radiation budget,
including shortwave and longwave
components
5.
Define and differentiate obliquity,
eccentricity, and precession
6.
Describe the cause of seasons on Earth
7.
Describe the diurnal cycle of radiative
fluxes
Introduction
Outside on a sunny day, you can feel the sun’s American
66 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorological Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy on your skin. You
can feel it in the form of heat due to the transfer of American
Meteorological Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy between objects.
Standing under the shade of a tree (which blocks the sun’s
rays) makes a significant difference in temperature and your
probability of getting a sunburn. The closest Earth gets to the
Sun is approximately 93 million miles. How does the sun’s
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy reach so far? The
answer is in American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation. American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">Radiation is the primary
mechanism of American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy transfer on Earth, including the
transfer of American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy from the Sun to the Earth over
great distances through the vacuum of space.
ATMO 200 • 67
A setting sun with an orange sky shining over a breaking ocean wave
(CC BY 2.0).
Radiation
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">Radiation can be thought of in two ways:
electromagnetic waves or as photons. For the purpose of
atmospheric science, we will generally consider American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation as a wave
rather than a photon particle. American Meteorological
Society, cited 2019: Electromagnetic radiation. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Electromagnetic_radiation.]"
class="glossaryLink">Electromagnetic radiation is a type of
68 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy produced by
electric and magnetic fields, taking a variety of names depending
on the wavelength. For example, you’ve probably heard of radio
waves and x-rays, whereby the primary difference is the
wavelength of the waves in each. The next image shows the
relationship between wavelength, frequency, and temperature.
We will go into further detail later, but colder objects radiate at
lower frequencies and longer wavelengths, and warmer objects
radiate at higher frequency and shorter wavelengths. This
relationship holds true across all scales, from atomic nuclei to
planets, and across temperatures from near absolute zero to 10s
of millions of Kelvins.
The electromagnetic energy spectrum, including penetration of Earth’s
atmosphere (top), wavelength visual (red line), radiation type (name
given), wavelength value (m), wavelength scale (image), frequency
(Hz), and temperature of black body objects that emit at that wavelength
of radiation (CC BY-SA 3.0).
ATMO 200 • 69
Wave Propagation
How fast does the Sun’s American Meteorological Society, cited
2019: Electromagnetic radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Electromagnetic_radiation.]"
class="glossaryLink">electromagnetic radiation travel to Earth
and how can we characterize the waves? All electromagnetic
waves travel at the speed of light (often given the variable “c0“,
approximately 3×108 m·s–1). The waves have a wavelength (λ)
given by the distance from one wave crest to the next. The waves
also have a frequency (v), which is the number of repeated wave
occurrences in a specified period of time. The unit for frequency
is Hertz (cycles per second). Finally, one can also define a
wavenumber for waves, which is one over the wavelength, or the
number of waves in each meter, σ (cycles·m–1) = 1 / λ. Circular
frequency is defined as ω (radians·s–1) = 2π·ν.
With variables for wavelength and frequency, we can very
clearly define any type of wave.
Hawaii Focus Box
Electromagnetic waves are simply a type of wave.
They can be described with some of the same words
and variables as ocean waves. If you are a surfer, you
70 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
probably check the swell forecast for wave height
and wave period before you head out to the water.
The best waves for surfing have a long period, which
helps to separate and clearly define individual waves.
Wave periods larger than 12 or 14 seconds are ideal.
Depending on whether you are a beginner or
advanced, you might like to have a wave face height
between 2 and 30 feet, respectively.
A breaking ocean wave (NOAA Public Domain).
Wave period is defined as the time it takes for a
wave to complete one cycle. Wave period is simply
the inverse of wave frequency.
Wave face height is related to both wave
amplitude and frequency, but is much more
complicated because in the case of surfing, it is
measured as the wave breaks. In the case of
electromagnetic American Meteorological Society,
cited 2019: Energy. Glossary of Meteorology.
ATMO 200 • 71
[Available online at http://glossary.ametsoc.org/wiki/
Energy.]" class="glossaryLink">energy, the waves
do not break.
Characterizing Emission
Where does this electromagnetic American Meteorological
Society, cited 2019: Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy come from? It surrounds us every
moment of every day in many forms. In fact, any object warmer
than absolute zero (0 K) emits radiant American Meteorological
Society, cited 2019: Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy. In order to estimate the amount of
radiant American Meteorological Society, cited 2019: Energy.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy an object emits, a common
simplification is needed: we assume an object behaves as a
American Meteorological Society, cited 2019: Blackbody.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Blackbody.]"
class="glossaryLink">blackbody. A American Meteorological
Society, cited 2019: Blackbody. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Blackbody.]" class="glossaryLink">blackbody is an object
that emits and absorbs the maximum amount of American
72 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation possible
given its temperature.
Planck’s Curves
Planck’s curves are used to show the amount of emitted
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation and primary wavelengths of
electromagnetic American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy that a black body emits given its
temperature. The diagram below shows multiple Planck function
curves for various temperature black bodies. The red line
denotes an object that has a temperature of 3000 K, the green
line is 4000 K, and the blue line is 5000 K. As the black body
becomes hotter, it emits at shorter wavelengths and greater
intensities.
ATMO 200 • 73
The spectral radiance (y-axis, emitted radiation) vs. the wavelength
(x-axis) of radiation emitted from black bodies of various temperatures.
As the temperature of the black body increases, the amount of emitted
radiation increases, and the radiation is emitted at shorter and shorter
wavelengths (Public Domain).
Wien’s Law
Planck’s function tells us the amount of emitted American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]"
class="glossaryLink">radiation
and
wavelengths over which it is emitted given the temperature of
the black body. We can use another law to determine the
maximum wavelength emitted by a black body. American
Meteorological Society, cited 2019: Wien’s law. Glossary of
Meteorology.
[Available
online
at
74 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://glossary.ametsoc.org/wiki/Wien%27s_law.]"
class="glossaryLink">Wien’s Law states that the shorter the
wavelength emitted, the hotter (more American Meteorological
Society, cited 2019: Kinetic energy. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Kinetic_energy.]" class="glossaryLink">kinetic energy) the
object is. In Wien’s equation, sometimes the numerator is given
as “a”, a constant equal to 2897.
Using Wein’s equation to find wavelength gives an answer in
microns, µm. One micron is equal to 10-6 meters.
2.1 Solar Radiation Wavelength
Question: Our sun is a yellow dwarf star with a
surface temperature near 5800 K. What is the
maximum wavelength of the American
Meteorological Society, cited 2019: Radiation.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation it emits? Where does
ATMO 200 • 75
that American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation fall on
the electromagnetic spectrum?
Answer: Wein’s Law gives us an equation that
relates temperature to the maximum wavelength of
American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation emitted
by an object:
This American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation, 0.499
µm, is in the range of visible light, given as 0.5 ×
10-6 m by the chart earlier this chapter.
Stefan-Boltzmann Law
One last very important law for American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation will be discussed
76 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
here. The American Meteorological Society, cited 2019:
Stefan-Boltzmann Law. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Stefanboltzmann_law.]" class="glossaryLink">Stefan-Boltzmann
Law relates the total American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation emitted (total emitted power per
area) to the area under Planck’s curve. This can be used to show
that the hotter the object, the more American Meteorological
Society, cited 2019: Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy it radiates per unit area.
In the above equation the amount of radiance emitted per area
is equal to the temperature of the black body raised to the 4th
power. This relationship is extremely important. It shows that
the amount of American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation emitted depends heavily on
temperature such that small temperature fluctuations result in
large changes in emittance.
ATMO 200 • 77
The Stefan-Boltzmann constant is 5.67×10–8 W·m–2·K–4 and
the symbol sigma, σ, is used.
Pro Tip: Whenever you see the Stefan-Boltzmann
constant being used (σ), you know that the assumption
of a black body has been made.
Application to the Earth-Sun System
In the prior section, the discussion turned rather technical. We
went down a rabbit hole of American Meteorological Society,
cited 2019: Radiation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation wavelengths, total emitted
power per area, and the assumption of black bodies. We did
this so that we could understand basic relationships about the
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy balance in the
Earth-Sun system. The Sun’s average temperature is above
5,000 K while the Earth’s average temperature is in the range
210-310 K (we will discuss this further in a later chapter). This
means that the Sun and Earth radiate American Meteorological
Society, cited 2019: Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy very differently.
The Sun emits solar American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
78 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">radiation, also known as ultraviolet
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation or shortwave American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation. The Earth
emits infrared American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation or
longwave
American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation. This follows
directly from the electromagnetic American Meteorological
Society, cited 2019: Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy spectrum and the respective
temperatures of the Sun and Earth. The Sun emits American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation at a shorter
wavelength than the Earth because it has a higher temperature,
and Planck’s curve for higher temperatures peaks at shorter
wavelengths. It is for this reason that Earth’s American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation is referred to
as longwave, and the Sun’s American Meteorological Society,
ATMO 200 • 79
cited 2019: Radiation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation is called shortwave.
We learned that a black body absorbs all incoming American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation and emits the
maximum possible American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation given its temperature. In the
Earth-Sun system, this means that the Earth absorbs all incoming
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation from the Sun, and emits the
maximum amount given its temperature. In practice it is a little
bit more complicated than this.
Albedo
Not all incoming solar American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation is absorbed by the Earth
because the Earth is not a perfect black body. Instead of
absorbing all incoming American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
80 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">radiation, some of it is reflected.
Reflection refers to radiative American Meteorological Society,
cited 2019: Energy. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy bouncing back away from an
object. We can define American Meteorological Society, cited
2019: Albedo. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Albedo.]"
class="glossaryLink">albedo (α) as the ratio of the amount
of American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation reflected from an object to the
amount of American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation received by an object.
Pro Tip: The simplest way to think of American
Meteorological Society, cited 2019: Albedo. Glossary
of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Albedo.]"
class="glossaryLink">albedo is based on the color of
the object. Objects that are white colored are highly
reflective and have a high American Meteorological
Society, cited 2019: Albedo. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Albedo.]" class="glossaryLink">albedo because the
ATMO 200 • 81
amount of reflected light is large compared to the
amount of incoming incident light. You’ll know this if
you’ve been on a white sandy beach or a snow field
and felt the brightness of the environment. Following
the same logic, dark surfaces absorb light so black,
brown, or dark green surfaces typically have a low
American Meteorological Society, cited 2019: Albedo.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Albedo.]"
class="glossaryLink">albedo. You’ll know this if
you’ve walked across a black paved surface and felt
the temperature difference between the blacktop and
the white painted parking space demarcations.
Earth has an American Meteorological Society, cited 2019:
Albedo. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Albedo.]"
class="glossaryLink">albedo of about 0.3 or 30%. This is an
average that takes into account the high American
Meteorological Society, cited 2019: Albedo. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Albedo.]" class="glossaryLink">albedo and high latitude
regions that are snow covered as well as the clouds and the much
lower American Meteorological Society, cited 2019: Albedo.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Albedo.]"
class="glossaryLink">albedo oceans. Note that American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation reflected from
an object does not warm the object.
82 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Surface Energy Balance
Arguably the most important aspect to consider about the EarthSun system is the American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy
balance.
In
American
Meteorological Society, cited 2019: Steady state. Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Steady_state.]"
class="glossaryLink">steady-state, the amount of incoming
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy should equal the
amount of outgoing American Meteorological Society, cited
2019: Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy (Net Radiative Flux=F*=0).
Let’s start with the incoming solar American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation. The solar constant
“S” is approximately equal to 1361 W·m-2. This value is a rough
estimate of the amount of American Meteorological Society,
cited 2019: Energy. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy per area received by the Earth
from the Sun, but it is not exact. We call it a solar “constant”
but it can be slightly lower or higher at times. American
ATMO 200 • 83
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">Radiation emitted from
a spherical source, like the sun, decreases by the square of the
distance from the sphere’s center. This is called the inverse
square law.
The following equation is a basic budget equation. Net
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation (F*) is equal to the incoming
solar American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation (K↓), the reflected solar
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation (K↑), the longwave American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation emitted by the
Earth (I↑), and the downwelling longwave American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation emitted from
the atmosphere (I↓) received by the Earth’s surface.
84 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Some of the sunlight that reaches the surface of the Earth is
reflected, depending on the American Meteorological Society,
cited 2019: Albedo. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Albedo.]"
class="glossaryLink">albedo, K↑ is often written in the
following way:
This is a brief introduction to a surface American
Meteorological Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy balance model. As
you may imagine, it can become much more complicated
depending on the factors involved. It also strongly depends on
the number of layers considered in the model. We will discuss
this further in a later chapter.
ATMO 200 • 85
Earth’s radiative balance, incoming and outgoing radiation. (NASA
Public Domain). Description in the text below.
For now, you should understand that incoming solar American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation is called
shortwave American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation and is in the ultraviolet and
visible portions of the electromagnetic spectrum because of the
emission temperature of the Sun. When solar American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation interacts with
the Earth, it is partially absorbed by the Earth’s surface, and
partially reflected, depending on the American Meteorological
Society, cited 2019: Albedo. Glossary of Meteorology.
86 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
[Available online at http://glossary.ametsoc.org/wiki/Albedo.]"
class="glossaryLink">albedo of the surface. In the diagram
above, you can see that some of the incoming solar American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation is reflected by
clouds, some is reflected by the Earth’s surface, but most is
absorbed by the Earth’s surface or the atmosphere.
You should also understand that Earth emits American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation too. However,
it is at a lower intensity and a much longer wavelength, which
is called the infrared portion of the electromagnetic spectrum
because of the lower emission temperature of the Earth.
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">Radiation is emitted by the Earth’s
surface, and by the atmosphere. We’ll go into more detail on this
later.
Pro Tip: Many synonyms were discussed above.
When referring to American Meteorological Society,
cited 2019: Radiation. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation from the
sun you may hear the following words used: solar
American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online
ATMO 200 • 87
at http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation, shortwave American
Meteorological Society, cited 2019: Radiation.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation, and ultraviolet
American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation. When referring to
American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation from the Earth you
may hear the following words used: infrared American
Meteorological Society, cited 2019: Radiation.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation, longwave American
Meteorological Society, cited 2019: Radiation.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation.
Earth’s average temperature remains relatively constant as there
is a balance of outgoing American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation and incoming American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation.
88 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Radiation Interaction with the Atmosphere
The diagram below shows the spectral intensity of downgoing
solar American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation (UV and visible in red) and
upgoing thermal American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation (IR in blue). The panels below
the spectral intensity graph show the total percentage of
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation absorbed and scattered by the
atmosphere as a function of wavelength, and divided up by
the primary American Meteorological Society, cited 2019:
Greenhouse gases. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Greenhouse_gases.]"
class="glossaryLink">greenhouse gases in the atmosphere.
American Meteorological Society, cited 2019: Greenhouse
gases. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Greenhouse_gases.]"
class="glossaryLink">Greenhouse gases are gases in the
Earth’s atmosphere that can absorb and emit American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation, primarily
infrared American Meteorological Society, cited 2019:
ATMO 200 • 89
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation.
The greenhouse gas that interacts with American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation across the largest
number of wavelengths is water vapor (H2O), which interacts
strongly with the atmosphere, especially in the longer
wavelength portion of the spectrum. Other gases, such as Carbon
Dioxide (CO2), Oxygen (O2), Ozone (O3), Methane (CH4), and
Nitrous Oxide (NO) also interact with American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation.
90 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Radiation transmitted by the atmosphere as a function of wavelength.
Downgoing solar radiation is on the left, and upgoing thermal radiation
is on the right, with the percentage of radiation absorbed and scattered
by the Earth’s atmosphere below. Finally, the percentage of radiation
absorbed and scattered is then divided by the major greenhouse gases in
the panels at the bottom (CC BY-SA 3.0).
In the panel of the figure above showing the total percentage
of American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation that is absorbed by the Earth’s
ATMO 200 • 91
atmosphere, you’ll notice white gaps. These are called
American Meteorological Society, cited 2019: Atmospheric
Window. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Atmospheric_window.]"
class="glossaryLink">Atmospheric Windows where the
wavelength of American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation is able to pass through the
atmosphere without interaction with American Meteorological
Society, cited 2019: Greenhouse gases. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Greenhouse_gases.]"
class="glossaryLink">greenhouse
gases. Particularly notable is the visible portion of the
electromagnetic American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy spectrum that can pass through
the atmosphere unimpeded. Also notable is the large amount
of interference in the infrared portion of the spectrum. Earth’s
atmosphere is practically transparent for shortwave American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]" class="glossaryLink">radiation, and it
strongly absorbs infrared American Meteorological Society,
cited 2019: Radiation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation. This will be important for
American Meteorological Society, cited 2019: Climate. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
92 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
wiki/Climate.]" class="glossaryLink">climate factors discussed
in a later chapter.
Open-ended Question: Why do you think visible
light (the peak of down-going solar American
Meteorological Society, cited 2019: Radiation.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation) is centered on an
atmospheric window?
The interaction of atmospheric gases in Earth’s atmosphere is
an important part of its American Meteorological Society, cited
2019: Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy balance. The greenhouse effect is
the result of American Meteorological Society, cited 2019:
Greenhouse gases. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Greenhouse_gases.]"
class="glossaryLink">greenhouse gases absorbing and emitting
outgoing infrared American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation emitted from Earth’s surface.
Because Earth’s outgoing longwave American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation is partially
absorbed by the atmosphere, this has a warming effect on Earth’s
surface making it warmer than it would be otherwise. While the
ATMO 200 • 93
greenhouse effect has a bad reputation, this process has in fact
allowed Earth to be habitable.
Radiation Changes with Time
The amount of American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation the Earth receives from the Sun
varies over many time scales, from thousands of years, to one
year, to daily time periods. These will be discussed in the
following sections.
Changes in solar American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation on both daily and annual time
scales can be explained by Earth’s orbital path around the Sun.
These aspects will be discussed more thoroughly in a later
chapter, but for now, let’s have a brief introduction. There are
three primary factors to consider: eccentricity, obliquity, and
precession. Eccentricity is the circularity of a planetary orbit.
For example, a circle has zero eccentricity. Obliquity is the
degree of tilt in the axis of rotation. Finally, precession is the
wobble in the rotational axis of a planet that slowly traces out
a cone. The three orbital parameters are shown in the image
below.
94 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Visual displaying three orbital parameters: eccentricity, obliquity, and
precession (NASA Public Domain).
Currently the Earth has a 0.0167 orbital eccentricity, 23.44
degree tilt from vertical, and precesses on very long time scales.
In fact, all of these factors change slightly over very long time
scales but for now, let’s consider them to be constant.
Seasonal Changes
Seasons occur due to changes in solar American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation that come from the
position of Earth with respect to the Sun. The diagram below
shows the position of the Earth with respect to the sun during
each of the four seasons.
ATMO 200 • 95
Schematic diagram showing the reason Earth has seasons (CC BY-SA
4.0).
During summertime in each hemisphere, the hemisphere is
facing toward the sun. For example, in June in the Northern
Hemisphere summer, the sun shines more directly on the
Northern Hemisphere than the Southern Hemisphere. While it
seems like a small change, the sun angle and the amount of
96 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
solar American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation absorbed (per area) varies
significantly throughout the year. This is because in addition to
changing the angle of the Sun, the position of the Earth also
changes the length of day throughout the year. Seasons are due
to the tilt of the Earth. If the Earth’s rotation was not tilted, it
would not have seasonal changes.
Pro Tip: The cause of seasons on Earth is the topic
of a common misperception. Earth’s seasons are
caused by its tilted orbit. While it is true that Earth’s
orbit is not spherical, and it is closer to the Sun during
Northern Hemisphere winter, the distance from Earth
to the Sun is not the cause of seasons.
The diagram below shows the summer season in the Southern
Hemisphere where there is a higher density of incident rays
due to the higher Sun angle. The Northern Hemisphere is
experiencing wintertime. When the Sun’s rays are at an angle
as they are in the Northern Hemisphere, the same amount of
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy is spread over a
larger area than they would be if the Sun’s rays were
perpendicular to the surface. Again, the angle of the Sun’s rays
and the length of day change because of the Earth’s tilt.
ATMO 200 • 97
Distribution of solar rays on Earth, with the summer season
receiving the majority of the solar radiation (Public Domain).
Daily Changes
Daily changes are also called “diurnal.” The Earth absorbs
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation from the Sun during the
daytime. This is only true for one location, but accounts for
the increase in temperature throughout the day from a point
perspective. What we likely have not experienced directly is that
the Earth emits infrared American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation all day and night. The lack of
incoming solar American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
98 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">radiation and the emission of infrared
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation is what accounts for the
decrease in temperature at night. The combination of nonstop
outgoing longwave radiational cooling from Earth’s radiative
emissions and daytime solar shortwave radiational heating
results in a diurnal cycle of net American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation and temperature, as
seen below.
The amount of radiative energy (y-axis, in W m-2) as a function of time
(x-axis, in hours). Yellow shows net warming and blue shows net
cooling. The temperature is coolest at sunrise, warms during the day and
reaches its maximum temperature in the late afternoon. (Image Created
by Britt Seifert).
ATMO 200 • 99
In this chapter we focused on the American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation: how we can define
and describe it by wavelength and intensity; the temperatures it
corresponds to as well as how it changes over time scales. As a
reminder, these were our learning goals:
1. Define black body American Meteorological Society,
cited 2019: Radiation. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation and
Planck’s Law
2. Apply American Meteorological Society, cited 2019:
Wien’s law. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/
Wien%27s_law.]" class="glossaryLink">Wien’s law
to compute the maximum emission wavelength
3. Use Stefan-Boltzmann’s law to compute radiative
emittance
4. Describe Earth’s surface American Meteorological
Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation budget, including
shortwave and longwave components
5. Define and differentiate obliquity, eccentricity, and
100 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
precession
6. Describe the cause of seasons on Earth
7. Describe the diurnal cycle of radiative fluxes
In the following chapter, we’ll begin to see how this American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]"
class="glossaryLink">radiation
does
American Meteorological Society, cited 2019: Work. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Work.]" class="glossaryLink">work, and what that means
for the environment.
Chapter 2: Questions to Consider
1.
Are the following statements about
American Meteorological Society, cited
2019: Radiation. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Radiation.]"
class="glossaryLink">radiation true or
false?
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
ATMO 200 • 101
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=72
2.
Describe the relationship between
wavelength, American Meteorological
Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Energy.]" class="glossaryLink">energy
emission, and the temperature of an
object.
3.
If the temperature of the Earth is 257
K, what is the total radiative flux
emitted? What is the peak emission
wavelength?
4.
How would the seasons change if the
Earth’s tilt were greater?
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/atmo/?p=72
Chapter 3: Thermodynamics
ALISON NUGENT AND DAVID DECOU
Learning Objectives
By the end of this chapter, you should be able to:
1.
Define and describe four methods of
energy transfer
2.
Describe the change in energy
associated with changes in water state
3.
Define and apply the first law of
thermodynamics
4.
Differentiate Eulerian and Lagrangian
frameworks
5.
Describe the importance of the dry
102
ATMO 200 • 103
adiabatic lapse rate, and recall what sets
its constant value in the atmosphere
6.
Compute potential temperature and
apply the conserved variable approach
7.
Draw a diagram of surface heat fluxes
and Earth’s radiation budget
8.
Compute the Bowen ratio, and define
latent and sensible heat flux
Introduction
What is Energy? You may hear frequently about “green
energy”, “clean energy”, “renewable energy”, and “solar
energy” in the media, as energy is currently a hot topic. Power
plants, wind turbines, and solar panels may come to mind. The
first paragraph in Roland Stull’s Practical Meteorology Chapter
3 discusses several types of energy right off the bat, but first of
all, what is energy?
104 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Convective clouds in the atmosphere are driven by an enormous source
of energy called latent heat (CC BY-SA 2.0).
Your intuition will probably tell you that we need energy in
order to do things. Energy makes stuff “go”. Your appliances,
your car, and your body all need energy. Your body utilizes
energy to maintain its various functions even as you read this
sentence. You know that energy is a real thing that exists; you
see evidence of it everywhere. The heat from your stovetop, the
ice melting in your glass, and that thunderstorm rolling in are
all evidence of energy at work, on scales both micro and macro.
Energy can be defined as the ability to do work. When you
apply a force on an object, it is said that work is done on the
object if that object is displaced, meaning it moves from its
original location. For example, when you pick up a book, you
exert a force against gravity causing the book to change
position, and you do work on the book. The higher you lift the
book or the further you throw it (should you decide to), the
ATMO 200 • 105
more work you do. However, if you apply a lot of force on a
heavy piece of furniture but it doesn’t move from its original
place, no work has been done regardless of the amount of
effort.
Note: ∆ is the Greek letter delta, and denotes a
change. Here, ∆d is the change in position, or the
displacement. The standard unit of force is a Newton
(N = kg·m·s-2), which is defined as the force required
to make a mass (m) of 1 kg accelerate at 1 m·s-2.
Force (F) is equal to the mass (m) of an object times
its acceleration (a): F = m*a.
Internal energy is the total amount of energy stored in any
object and determines how much work the object is capable of
performing. This includes both kinetic energy (energy that an
object has when it is in motion) and potential energy (energy
that is stored). For example, a bowling ball sitting on a table
contains energy despite the fact it is not in motion. It does not
have kinetic energy because it is still, but it contains potential
energy simply because of where it is situated. Were it nudged
off the table, the bowling ball will do work because it will be
pulled downward by gravity. This is an example of
gravitational potential energy. The potential energy (PE) due to
gravitational pull is given by the following equation:
106 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Gravitational potential energy (CC BY 2.0).
Anything that moves contains kinetic energy (KE), which is
given by the following equation
where m is the mass of an object in kilograms (kg) and v is the
velocity of an object in meters per second (m·s-1). From this
relationship you can see that objects with more mass or objects
that are moving faster have more energy.
Energy takes on many forms and often changes forms from one
to the other, but the total amount of energy in the universe
remains constant. Energy cannot be created or destroyed. This
means that the energy lost in a process must be the same as the
ATMO 200 • 107
energy gained in another. This is what the law of conservation
of energy means, and this is what is known as the first law of
thermodynamics. The first law of thermodynamics frequently
comes into play in atmospheric motions and will be discussed
further later in this chapter.
In short, energy is the capacity of a system to perform work.
Energy is always conserved and cannot be created or destroyed.
We begin this chapter with a short review of energy because
energy is ultimately responsible for Earth’s weather from
temperature changes in the atmosphere to the resulting air
motions. Without energy, no weather would occur.
Energy Transfer
One important example of kinetic energy is thermal energy,
which comes from the tiny movement of many molecules in a
system. In Chapter 1, we discussed how temperature is a
measure of the average speed of atoms and molecules in a
system. Here we can further describe temperature as being
proportional to the average kinetic energy of the random
motions of the molecules in a system. The faster the molecules
move, the higher the temperature.
The transfer of thermal energy due to the temperature
difference between two objects is what is known as heat. Heat
is a form of energy in transit, and once transferred, it is stored
as internal energy. There are four main methods of heat
transfer: conduction, convection, radiation, and the absorption
or release of latent heat.
108 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Conduction, convection, and radiation are all methods of heat transfer
(CC BY-SA 4.0).
When you directly touch a hot object, such as the stovetop, the
energy from the hot stove top is immediately transferred to your
cool hand due to a difference in the speed of the molecules,
causing you to feel a burn. This is an example of conduction:
energy directly transferred through a substance without the
movement of material. Certain materials are better conductors
of heat than others. Metal, for example, conducts heat very
efficiently, while air, which acts as an insulator, is a very poor
conductor of heat.
Another type of energy transfer is convection, which occurs in
liquids and gases (both fluids) because molecules can move
freely and currents can naturally occur in them. This happens
constantly in the atmosphere. Convection refers to movement
within a fluid due to the tendency of lower density fluid to rise
over higher density fluid, which sinks due to the force of
gravity resulting in heat transfer within the fluid.
ATMO 200 • 109
In the figure below, a beaker is being heated from the bottom by
a flame. The arrows show upward movement in the center
where the fluid is heated and therefore is less dense and
buoyant. The cooler fluid at the top of the beaker is more dense
and sinks toward the bottom under the influence of gravity.
Arrows show the motion of
convection within a fluid that is heated
from the bottom (CC BY 2.0).
Finally, when it comes to the atmosphere, the most important
form of energy is the energy we get from the Sun, which is
called radiant energy or radiation. Radiation is another type of
heat transfer that was covered in Chapter 2. Remember that
radiant energy can be transferred to an object without the space
in between being heated. This is the result of electromagnetic
waves from the Sun, which are then absorbed by the Earth’s
110 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
surface or atmosphere and converted to thermal energy.
Electromagnetic waves do not need any matter to travel through
and propagate at the speed of light in a vacuum which is about
300,000 km·s–1.
Heat, or the transfer of thermal energy due to the difference in
temperature between two objects, is defined as ∆q and is given
in standard units of joules per kilogram (J·kg–1). One joule (J)
is the standard unit of energy or work (kg·m2·s-2).
Latent Heat
Latent heat refers to the amount of heat that is added or
removed from a system when a change of phase takes place. In
atmospheric science, this almost always refers to the phase
changes of water (H2O).
When heat is added to an ice cube, the temperature of the ice
will increase until it reaches its melting point (0°C). After this
point, any further addition of heat will cause the ice to melt to
liquid water, but the temperature of the ice-water system will
not change until the phase change is complete. The heat that is
absorbed by the ice-water system from the environment in order
for the phase change to occur is known as latent heat.
You have experienced this as your ice melts in your water glass.
It stays cold until all of the ice is gone and then rapidly warms.
The absorption of latent heat is also the reason you feel cold
right after leaving your shower or a body of water. The excess
water on your skin starts to evaporate, but it requires additional
energy in order to transition from liquid to vapor. Energy is
ATMO 200 • 111
absorbed from the environment (your skin) to evaporate the
remaining liquid, causing your skin to cool. This is known as
evaporative cooling.
Processes that take heat from the environment (melting,
evaporation, sublimation) are considered cooling processes. On
the other hand, processes that add heat to the environment
(condensation, freezing, deposition) are warming processes
because during the phase change energy is released. This is an
important distinction, so be sure this is clear before moving on.
I’ll repeat it below for emphasis.
Pro Tip: Melting, evaporation, and sublimation are
considered cooling processes because they take heat
from the environment. Condensation, freezing, and
deposition are considered warming processes because
during the phase change energy is released and they
add heat to the environment.
112 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Phase changes of water (CC BY-SA 3.0). The lower energy state is on
the left, and the higher energy state is on the right. Going from left to
right takes energy (cools the environment), going from right to left
releases energy (warms the environment).
The reason phase changes of water are so important in the
atmosphere is because it acts as the energy to fuel convection
and thunderstorms. When water vapor condenses to form the
liquid water found in clouds, this process releases enormous
amounts of latent heat to the environment.
Latent heat (QE) is the latent energy that is possessed by an
object of mass m, but usually we want to know the change of
latent heat that is caused by a phase change process.
The change in latent heat is equal to the latent heat factor
multiplied by the mass of water undergoing a phase change.
ATMO 200 • 113
The y-axis of this figure shows the state of water at different
temperatures. The x-axis shows the amount of energy applied to the
water. When water reaches 0°C and 100°C, the melting/freezing and
boiling points of water, respectively, the temperature stops increasing
while energy is still being applied. Instead of increasing the water’s
temperature, this energy is instead going into the processes of melting
and evaporation. From here, you can see that the latent heat of
vaporization is much greater than the latent heat of melting, because
much more energy is applied while the temperature remains the same
(CC BY-NC-SA 4.0).
Different phases of water have different latent heat factors (L),
meaning the change in phase of a mass of water corresponding
to the change in latent heat is different depending on what
process is taking place. This is well displayed in the image
114 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
above by the horizontally flat regions where energy is going
into the system but no temperature change is occurring.
For evaporation or condensation (phases changes between
liquid water and vapor), the latent heat of vaporization, Lv = 2.5
* 106 J·kg–1 is used. For melting or freezing (phase changes
between ice and liquid water), the latent heat of fusion or
freezing, Lf = 3.34 * 105 J·kg–1 is used.
Specific Heat
Latent heat is the term for the amount of heat released from
changes in phase, but what about changes in temperature
unassociated with phase changes? These portions are the
slanted lines in the figure above where energy is going into the
system and the temperature is responding. Specific heat or
heat capacity (C) is the term for the amount of heat required to
raise the temperature of a substance by a certain amount.
Typically this is defined at a constant pressure (Cp) or a
constant volume (Cv).
Some substances, such as water, require a particularly large
amount of heat energy in order for the temperature to change.
The heat capacity of a substance is the ratio of heat energy that
is absorbed to the corresponding rise in temperature. The
specific heat is the heat capacity of a substance per a unit mass.
Basically, specific heat is the amount of heat that is needed in
order to raise the temperature of one gram (g) of a substance by
one degree Celsius, or Kelvin.
For example, the specific heat of pure water is 4186 Joules per
ATMO 200 • 115
degree Kelvin per kilogram (J·K-1·kg-1), while the specific heat
of dry air at sea level (Cpd) is about 1004 J·K-1·kg-1. The
specific heat of dry air at constant volume (Cvd) is equal to 717
J·kg–1·K-1. This means that it takes much more energy to raise
1 kg of water by 1°C than it does for 1 kg of dry air. Remember
that Celsius and Kelvin have a linear relationship so the change
in temperature of one degree Celsius is the same as a change of
one degree Kelvin (K = °C + 273).
Hawaii Focus Box
The difference in specific heat capacity of water
and land leads to different annual cycles of air
temperature and water temperature in Hawaii. The
heat capacity of sea water is about 3985 J·K-1·kg-1
while the heat capacity of land is typically less than
1000 J·K-1·kg-1. This means that land heats up faster
and cools down faster while the ocean takes longer to
warm and also takes longer to cool.
The warmest temperatures in Hawaii typically
follow the solar cycle with a slight lag. The warmest
temperatures in the Northern Hemisphere occur
during the time periods where the Sun’s angle is
highest and the length of day is longest as we
discussed in Chapter 2. This typically corresponds to
June through August. However, the warmest ocean
temperatures lag the land temperatures significantly
by a few months because water takes more energy to
warm. The warmest ocean temperatures occur from
August through October.
116 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
There is some feedback where the warm ocean
temperatures affect land temperatures as well, but it
all comes down to the difference in specific heat.
First Law of Thermodynamics
As stated previously, the first law of thermodynamics states that
energy is neither created nor destroyed. The total amount of
energy must be conserved. In the atmosphere, this means that
the amount of heat applied to a mass of air (thermal energy
input) must equal the total sum of the warming of the air, plus
the amount of work done per unit mass of air.
When heat (∆q, J·kg–1) is added to a mass of air (often referred
to as an air parcel, see below), some of the added thermal
energy warms the air, which increases its internal energy. When
the air parcel warms, one of two things must happen. Recall the
ideal gas law from Chapter 1: P=ρ*Rd*T. If temperature
increases, (1) density must decrease to keep pressure constant
or (2) the pressure will increase. Small pressure differences in
the atmosphere always equalize first, so as the air warms, the
density of the air parcel decreases. The decreased density of the
air parcel gives it a lower density than the surrounding air, and
it is therefore buoyant and begins to rise. As the parcel rises, it
expands in volume in order to maintain equilibrium with the
lower pressure outside the air mass, and it pushes into the
surrounding atmosphere. Because of this, some of the thermal
energy that is added to the air goes into doing the work of
expansion and not all of it is used for warming.
ATMO 200 • 117
For the atmosphere, a more usable form of the First Law of
Thermodynamics is:
In the atmospheric form of the First Law of Thermodynamics,
Cp*ΔT can also be redefined as Enthalpy (h), a term to quantify
the total heat content of an air parcel.
This gives us the first term in the above equation for the first
law of thermodynamics.
Frameworks for Understanding the Atmosphere
All atmospheric science concepts are shared with other
disciplines like physics and chemistry, but they often use a
more specific set of equations or variables as shown above with
118 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
the first law of thermodynamics. This helps to simplify things
because general physics equations can be applied to nearly any
form of matter, while in atmospheric science we primarily deal
with gases that are not constrained to a specific volume.
In the two sections that follow, two important frameworks are
described that will help you to view the atmosphere in ways
that will simplify concepts in the sections and chapters to come.
Lagrangian vs. Eulerian
When we look at or try to solve a problem in the atmosphere,
there are two different lenses or frameworks through which we
can view the problem. An Eulerian framework is a fixed
framework, relative to a single point on the Earth’s surface.
When a weather forecast is done for a given location on Earth
or when you look at a dataset from one weather station, you are
viewing the atmosphere from an Eulerian perspective — that is,
how the wind and air travels past a fixed point. With a Eulerian
framework, we need to be concerned about things like
temperature and moisture advection, properties that travel with
and are carried by the wind.
Another framework is Lagrangian, which is a framework that
is constantly moving and travels with the air. When we are
looking at motions within the atmosphere, such as rising or
sinking air, it is useful to use this framework as a way to see
how properties within the rising plume of air are
changing. Both frameworks will be used in the coming sections
and chapters.
ATMO 200 • 119
Air Parcels
At this point, we should discuss a naming convention used in
atmospheric sciences. Many times it is useful to think about a
mass of air instead of individual molecules. We generally call a
mass of air an “air parcel”. This is especially useful for
distinguishing processes happening within the air, versus
processes happening within the environment. An air parcel is
often thought of as an amorphous bubble or blob of air, roughly
the scale of a party balloon or a hot air balloon that contains
uniform properties (temperature, density, pressure) throughout.
Air parcels are simplified theoretical constructions used as a
way to think about and examine motions and instability in the
atmosphere.
For example, let’s revisit the important topic of latent heating.
We discussed how condensation of water vapor to liquid water
is a warming process. This is only clear if we separate the
processes happening within a parcel of air and within the rest of
the environment. When water vapor condenses within an air
parcel, it gives off energy, which acts to warm the environment.
We will often refer to air parcels when we want to distinguish
changes within a piece of air with respect to the rest of the
atmosphere.
In reality, as air parcels move through the atmosphere, there
will be some mixing between the air parcel and the
environment, and heat can be exchanged with the environment
or added via radiation. However, with the concept of air
parcels, we simplify the situation and imagine that radiative
120 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
effects are small, and that mixing only occurs on the outer
edges of the parcel, with a protected inner core.
Defining Changes in the Atmosphere
In order to study changes in temperature, momentum, and
moisture in an air parcel, a Lagrangian framework is always
used. With a Lagrangian framework, we can see changes from
the parcel’s perspective as it moves, not from a fixed point on
the ground.
Applying the hydrostatic equation from Chapter 1 to the first
law of thermodynamics, we can get the Lagrangian first law of
thermodynamics equation for a moving air parcel.
∆q, or heat transfer, can be caused by various processes,
outlined in the following figure. The figure shows an air parcel
moving both horizontally (through advection) and vertically
(through convection), as well as the various processes both
inside and outside that are causing heat exchange. The
temperature inside the air parcel is conserved unless heat is
transferred to or from the environment, or if it loses or gains
heat by rising or sinking, which will cause it to expand or
contract, respectively.
ATMO 200 • 121
The effects of surroundings on air parcel temperature and the
corresponding heat exchange (CC BY-NC-SA 4.0).
Lapse Rates
The atmospheric lapse rate, Γ, denoted by an upper-case
gamma, is defined as the change in temperature with altitude,
specifically a reduction in temperature with altitude.
122 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
A positive lapse rate indicates that the temperature is decreasing
with increasing height, while a negative lapse rate indicates a
“temperature inversion” meaning that the temperature is
increasing with increasing height. Lapse rates are typically
defined for:
1. The environment, Γe
2. A dry air parcel, Γd
3. A saturated air parcel (moist), Γm
We can observe the environmental lapse rate by using
atmospheric sensors attached to weather balloons. As a weather
balloon rises through the atmosphere, it measures temperature
and other properties on the way. The environmental lapse rate
varies depending on time of day, altitude, latitude, land surface
properties, heat fluxes, and air movement. A typical Γe is
around 6.5 K km-1 but this will be discussed in later chapters.
You will have experienced this decrease in temperature with
altitude if you have hiked in the mountains or seen an outdoor
thermometer reading on a commercial flight.
Dry Adiabatic Lapse Rate
In atmospheric science, you will often hear the word
“adiabatic”, typically accompanied by the words “cooling”,
“warming”, or “lapse rate”. An Adiabatic process just means
that there is no heat transfer taking place (∆q = 0) during the
ATMO 200 • 123
process. For an air parcel, this means that no thermal energy is
entering or leaving the air parcel from the outside. However,
internal processes are allowed, such as the ones shown in the
figure above (most notably adiabatic expansion and latent
heating). An adiabatic lapse rate indicates that air is cooling or
warming with altitude without any external heat exchange. An
air parcel that contains no liquid water or ice (none of the
moisture in the parcel has condensed into liquid, no saturation
or latent heat release), will cool at the dry adiabatic lapse rate.
Pro Tip: A positive lapse rate means that the air
temperature is decreasing with height.
The reason that an air parcel expands adiabatically as it rises is
due to the fact that the environmental air pressure decreases
with height. The air parcel’s pressure will adjust to the lower
pressure of its environment, but the parcel must expand in order
to do so (in order for the air molecules inside the parcel to exert
a smaller force). The parcel uses some of its own internal
energy to do the work of expansion, and its temperature
decreases as a result. The opposite is true as an air parcel sinks,
the environmental pressure rises, and the parcel’s pressure
adjusts in order to maintain pressure equilibrium, and the parcel
must shrink for its pressure to increase. As it shrinks, work is
done on it, and the temperature rises.
In later chapters we’ll define the moist or wet adiabatic lapse
124 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
rate (Chapter 4), as well as the environmental lapse rate
(Chapter 5) and their significance.
Potential Temperature
The potential temperature, θ, of a parcel completely ignores
the temperature change of the parcel due to it having done work
or been worked on (expanding or contracting). As a result,
potential temperature is constant for an adiabatic process and θ
does not change when ∆q = 0. Potential temperature is
proportional to the sensible heat contained in a parcel and can
increase or decrease when sensible heat is added or removed
through diabatic (non-adiabatic) processes (∆q ≠ 0). Examples
of diabatic heating include turbulent mixing, condensation
(latent heat), and radiative heating. Potential temperature has
units of K or °C, and can be found if you know the air
temperature, T, at pressure-level, P, using the following
equation
where Rd/Cp is a constant equal to 0.28571, and has no units,
and P0is a reference pressure, typically 1,000 hPa (100,000 Pa),
or the local surface pressure.
Introduction to Thermodynamic Diagrams
Potential temperature is also very useful in thermodynamic
diagrams, which will be briefly introduced here, but covered in
more detail in Chapter 5. Thermodynamic diagrams are useful
ATMO 200 • 125
in diagnosing the state of the atmosphere and the buoyancy of
air parcels by comparing the temperature difference ∆T
between the parcel and its environment. Parcels that are warmer
than their environment will tend to rise due to lower density,
and the change of the parcel’s temperature with height can be
anticipated based on the parcel’s moisture content. In the
thermodynamic diagrams you will use, dry adiabat lines will be
plotted to show the dry adiabatic lapse rate, as parcels will cool
at this rate until condensation occurs within the parcel. Dry
adiabats are labelled with θ because θ is constant along these
lines. A simple example is provided here.
126 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Sample thermodynamic diagram (CC BY-NC-SA 4.0). Horizontal lines
show isobars, lines of constant pressure; vertical lines show isotherms,
lines of constant temperature; and orange lines show dry adiabats, lines
of constant potential temperature.
Heat Budget at Earth’s Surface
Before moving on from thermodynamics, let’s add another
layer of complication to our understanding of Earth’s surface
heat budget. In Chapter 2 we discussed how the Earth’s heat
budget could be defined by the incoming shortwave radiation
(K↓), reflected shortwave radiation (K↑), longwave radiation
emitted by the Earth (I↑), and the downwelling longwave
ATMO 200 • 127
radiation emitted from the atmosphere (I↓) received by the
Earth’s surface.
Earth’s surface is considered to be infinitesimally thin with no
volume, and no heat can be stored, so the sum of all incoming
and outgoing heat fluxes at the surface must balance. The net
heat flux at the surface must be zero. In addition to the
incoming and outgoing shortwave and longwave radiation,
there are three other fluxes that must be considered. But first,
let’s discuss what is meant by a “flux” of heat.
Heat Flux
Let’s say you have a cube of air, somewhere fixed relative to
the ground. From an Eulerian framework (fixed-location),
pressure changes can be neglected in the First Law equation, as
they will be small and slow.
Leftover, you have an equation that states that the heat you
transfer to the cube (per unit mass) causes the temperature
change:
128 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
If you divide this by a time interval ∆t (note: lower-case t is
used for time, T is temperature), gives an equation for
temperature change with time:
The temperature of the air cube could be increased if there were
a heat transfer into it. Heat flux is the rate of heat transfer
through a surface over time, as illustrated in the below figure.
The units of heat flux can be given as J·m-2·s-1, or W·m-2,
because W = J·s-1. It is heat moving through an area over time.
This figure represents a flux of
some material or energy
through a surface or area (CC
BY-SA 3.0).
Temperature could be increased by heat flux into the cube of
air, and could also be decreased by heat flux out of the cube.
For example, if there is a net heat flux into a cube of air there
ATMO 200 • 129
will be a net heating effect because heat will be transferred into
the cube more quickly than it will leave the cube.
Earth’s Surface Budget
Fluxes are defined as positive for heat moving upward. In
addition to the net radiation (F*) from shortwave and longwave
radiation, the fluxes include:
F* = the net radiation between the surface and atmosphere,
defined above;
FH = effective surface turbulent heat flux (sensible heat flux,
SH);
FE = effective surface latent heat flux, caused by evaporation or
condensation (latent heat flux, LH); and
FG = molecular heat conduction to/from deeper below the
surface, basically heat being conducted from nearby molecules.
All of these fluxes have to balance.
Note: In the class we will likely use SH or SHF to
represent the sensible heat flux, and LH or LHF to
represent the latent heat flux. Remember, SHF is a dry
heat flux from convection and LHF is heat transfer
from moisture.
Over the course of a day, the relative contributions from the
different terms in the surface heat budget vary. The below
figure shows the four terms as they vary over a moist surface
during one average day. Notice the yellow line first. There is
130 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
negative F* through most of the day as the surface gains heat
from the excess shortwave radiation. At night, F* is positive as
the surface radiates infrared radiation upward away from the
surface. This is similar to FG, where heat is transferred
downward into the ground during the day and upward at night.
However FH and FE have opposite signs from F* and FG. The
surface sensible heat flux, FE and surface latent heat flux, FH
are positive during the day as convection and evaporation draws
heat upward away from the surface.
This figure shows the surface heat balance throughout the day over a
moist surface (CC BY-NC-SA 4.0).
Now let’s look at how the fluxes vary instantaneously over a
moist (a, b) or dry (c, d) surface during the day (a, c) and night
(b, d). The size of each arrow corresponds to the strength of
each type of flux.
ATMO 200 • 131
This diagram is a representation of Earth’s surface fluxes with different
132 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
sized arrows showing the magnitude of each under different conditions
(CC BY-NC-SA 4.0).
In the above image, note the difference between (a) and (c).
Both have a large incoming radiation F*, but the moist surface
(a) has a larger latent heat flux FE as compared to the dry desert
in (c) with a larger sensible heat flux FH. This shows that heat
is transferred from the ground to the atmosphere through
evaporation when the surface is moist, but through convection
when the surface is dry.
The Bowen ratio helps to distinguish various types of surfaces.
The Bowen ratio is defined as
the ratio between the sensible heat flux and the latent heat flux.
Moist surfaces have a small Bowen ratio because latent heating
dominates over sensible heating while dry surfaces have a large
Bowen ratio because sensible heating dominates over latent
heating.
What other surface heat flux changes do you notice
with day or night? How does energy partitioning
change depending on the available moisture?
Remember our learning goals for this chapter:
1. Define and describe four methods of energy transfer
2. Describe the change in energy associated with
ATMO 200 • 133
changes in water state
3. Define and apply the first law of thermodynamics
4. Differentiate Eulerian and Lagrangian frameworks
5. Describe the importance of the dry adiabatic lapse
rate, and recall what sets its constant value in the
atmosphere
6. Compute potential temperature and apply the
conserved variable approach
7. Draw a diagram of surface heat fluxes and Earth’s
radiation budget
8. Compute the Bowen ratio, and define latent and
sensible heat flux
Do you feel comfortable with all of the goals?
Additional Information
Temperature has been historically measured by a thermometer
like the one below. A bulb of expandable fluid grows or shrinks
depending on the temperature, indicating the temperature by its
top. Thermometers of this type are robust and cheap, but not
easily automated.
Liquid in glass thermometer (Public Domain).
134 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Another type of early thermometer records the minimum and
maximum with mercury (below). A marker is pushed upward
(for the maximum) or downward (for the minimum).
Minimum and maximum
mercury thermometer
(CC BY-SA 3.0).
Today, most thermometers are electrical. They use components
that are temperature sensitive, like a temperature sensitive
resistor or capacitor where the electrical current changes
depending on temperature. Based on the electrical reading from
the instrument, temperature can be automatically recorded.
ATMO 200 • 135
Electrical thermometers are used in systems such as this Elko
Automated Surface Observing System (ASOS) (CC BY-SA 3.0).
This type of thermometer is used in radiosondes, which are
launched on weather balloons for measuring temperature
throughout the entire atmospheric column. We can observe insitu as high as 50 km, halfway through the stratosphere with a
radiosonde balloon.
136 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Radiosondes contain thermistor temperature sensors (CC BY-SA 3.0).
Thermometers need to be correctly sheltered so that solar
radiation does not affect the temperature reading. A shelter is
typically placed at least 1 meter above the ground and covered
in a white housing that blocks the sun but allows air to flow
through like the one in the image.
ATMO 200 • 137
These meteorological shelters protect the instruments inside while still
allowing a fairly accurate collection of data (CC BY 3.0).
A temperature reading simply gives the temperature of the air,
but wind and humidity can affect the way the atmosphere feels
to the human body. The below image shows a wind chill graph
where the wind speed and the temperature gives an additional
measure of temperature called the wind chill. When the air
temperature is cold and the wind is high, the temperature feels
even colder and can give a person standing outdoors frostbite
within minutes (see the purple area below).
138 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Wind chill temperatures for corresponding dry bulb temperatures and
wind speed (Public Domain).
On the other hand when the air is humid, it feels hotter. This is
because when the air contains more water vapor and surfaces
(like your skin) cannot evaporate it efficiently. Unable to get rid
of heat through the latent heat flux (LHF), the surface heats up.
This is called the heat index. Notice that a temperature of 84°F
at 90% relative humidity has a heat index of 98°F.
ATMO 200 • 139
The heat index measures how hot it feels compared to the actual air
temperature by taking into account the humidity (Public Domain).
You have experienced at least one of these issues (wind chill or
heat index) in your lifetime.
Chapter 3: Questions to Consider
1.
Read the following scenarios and
chose the type of energy transfer
described.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=93
140 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
2.
Fill in the blanks to describe what
happens when an air parcel rises.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=93
3.
Earlier in the chapter, a brick being
held up was described as having potential
energy. If the brick weighs 3.5 kg and is
being held 1.2 m off the ground, how
much potential energy does it have?
Remember that acceleration from gravity
is 9.8 m⋅s-2.
4.
Mauna Kea, an inactive volcano on the
island of Hawai’i, has an elevation of
13,803 ft at the summit. If a weather
balloon measures the environmental
lapse rate to be 6.5 K⋅km-1, how much
cooler is the summit than the rest of the
island at sea level?
5.
In the winter, it often snows on top of
Mauna Kea. Explain how that’s possible
despite the mountain’s tropical location
using your result from the previous
question.
Selected Practice Question Answers:
ATMO 200 • 141
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/atmo/?p=93
Chapter 4: Water Vapor
ALISON NUGENT AND SHINTARO RUSSELL
Learning Objectives
By the end of this chapter, you should be able to:
1.
Compute American Meteorological
Society, cited 2019: Saturation. Glossary
of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Saturation.]"
class="glossaryLink">saturation
American Meteorological Society, cited
2019: Vapor pressure. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Vapor_pressure.]"
142
ATMO 200 • 143
class="glossaryLink">vapor pressure
using the Clausius-Clapeyron equation;
2.
Convert between humidity variables.
Differentiate between relative humidity,
specific humidity, absolute humidity,
wet-bulb temperature, mixing ratio, and
dew point;
3.
Describe the conditions for American
Meteorological Society, cited 2019:
Saturation. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Saturation.]"
class="glossaryLink">saturation to
occur;
4.
Apply the American Meteorological
Society, cited 2019: Moist adiabatic lapse
rate. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/
wiki/Moist-adiabatic_lapse_rate.]"
class="glossaryLink">moist adiabatic
lapse rate;
5.
Use the principles of phase change and
latent heating to describe why the
American Meteorological Society, cited
2019: Moist adiabatic lapse rate.
Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/
wiki/Moist-adiabatic_lapse_rate.]"
144 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">moist adiabatic
lapse rate is less than the American
Meteorological Society, cited 2019: Dryadiabatic lapse rate. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Dryadiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse
rate.
Introduction
Water can exist as a solid, liquid, or gas at typical conditions
found on Earth. As we learned, the process of liquid water
becoming water vapor is called evaporation and this process
absorbs or requires American Meteorological Society, cited
2019: Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy. The opposite process is called
condensation, where water vapor becomes liquid water,
releasing American Meteorological Society, cited 2019: Energy.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy. Condensation is especially
important in atmospheric science because this is the process that
allows clouds to form.
ATMO 200 • 145
Phase changes of water from gas (water vapor) to liquid (water) to solid
(ice) with the names for the processes also labeled. (CC BY 2.0).
Clouds are composed of millions and billions of tiny liquid water
droplets. How do they form? Why are they there?
Water droplets condensed on a glass surface (CC BY 2.0).
Before we can understand clouds in the atmosphere, we need
146 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
to explore concepts like how humidity is defined and what
American Meteorological Society, cited 2019: Saturation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation means.
In general, humidity is the amount of water vapor in the air.
You’ve likely heard of relative humidity and dew point
temperature, but what do these quantities mean physically?
Saturation
Imagine a closed jar filled halfway with water. At the initial
time, more water molecules evaporate from the water surface
than the number that return. However, after some time, the
number of molecules evaporating from the surface will be equal
to the number of molecules condensing back into the water
surface. When condensation and evaporation are equal, this is
called American Meteorological Society, cited 2019:
Saturation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation.
American Meteorological Society, cited 2019: Saturation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">Saturation occurs when air contains the
maximum amount of water vapor possible for its given
temperature. That is why condensation equals evaporation. If
ATMO 200 • 147
evaporation occurs, the air cannot contain more water vapor, so
some must condense. Now let’s get quantitative.
Vapor pressure at saturation
Every gas in the atmosphere exerts pressure, for example,
American Meteorological Society, cited 2019: Vapor pressure.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
class="glossaryLink">vapor pressure makes up a fraction of the
total atmospheric pressure. In the following equation, all of the
gases in Earth’s atmosphere contribute to the total atmospheric
pressure Patmosphere.
Specifically for water vapor, the more water vapor that is added
to the atmosphere, the higher the American Meteorological
Society, cited 2019: Vapor pressure. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Vapor_pressure.]" class="glossaryLink">vapor pressure PH2O.
The units for American Meteorological Society, cited 2019:
Vapor pressure. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
class="glossaryLink">vapor pressure are the same as pressure
and can be in Pascals, hectoPascals, or kiloPascals. Because we
are staying consistent with Roland Stull’s Practical Meteorology
textbook, we will use kiloPascals (kPa) throughout this chapter.
The amount of water vapor that the atmosphere can contain
depends on temperature. Lower temperature air cannot contain
148 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
as much water vapor as higher temperature air. If we think of this
quantitatively in terms of pressure, American Meteorological
Society, cited 2019: Saturation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Saturation.]"
class="glossaryLink">saturation
American
Meteorological Society, cited 2019: Vapor pressure. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Vapor_pressure.]" class="glossaryLink">vapor pressure
refers to the pressure exerted by the movement of water vapor
molecules exerted over a surface of liquid water. When the
partial pressure exerted by water vapor is equal to the American
Meteorological Society, cited 2019: Saturation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Saturation.]" class="glossaryLink">saturation American
Meteorological Society, cited 2019: Vapor pressure. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Vapor_pressure.]" class="glossaryLink">vapor pressure,
the air is saturated.
The Clausius-Clapeyron equation gives the approximate
relationship between American Meteorological Society, cited
2019: Saturation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation American Meteorological
Society, cited 2019: Vapor pressure. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Vapor_pressure.]" class="glossaryLink">vapor pressure (es) and
temperature in the atmosphere
ATMO 200 • 149
where the water-vapor gas constant Rv is 461 J·K–1·kg–1, T0 is
273.15 K, e0 is 0.6113 kPa, and Lv is the American
Meteorological Society, cited 2019: Latent heat. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Latent_heat.]" class="glossaryLink">latent heat of
vaporization, 2.5×106 J·kg–1. This results in Lv/Rv being equal
to 5423 K. In this equation, units for temperature must be in
Kelvin. Note that in the equation above, exp[x] implies the
exponential function ex, but it is written on one line for visual
purposes.
150 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
A graph of the saturation vapor pressure as a function of temperature
showing the exponential relationship between the two from the
Clausius-Clapeyron equation (Modified from CC BY-SA 4.0).
The image shows the relationship between temperature and
American Meteorological Society, cited 2019: Saturation.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation American Meteorological
Society, cited 2019: Vapor pressure. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Vapor_pressure.]" class="glossaryLink">vapor pressure based
on the Clausius-Clapeyron equation. Lower temperatures are
ATMO 200 • 151
saturated with respect to water vapor at lower vapor pressures,
while higher temperatures need higher vapor pressures to be
saturated. Temperature is the primary factor determining water
vapor American Meteorological Society, cited 2019: Saturation.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation.
In the graph of American Meteorological Society,
cited 2019: Saturation. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Saturation.]" class="glossaryLink">saturation
American Meteorological Society, cited 2019: Vapor
pressure. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
class="glossaryLink">vapor pressure vs. temperature
notice the American Meteorological Society, cited
2019: Saturation. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation American
Meteorological Society, cited 2019: Vapor pressure.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
class="glossaryLink">vapor pressure value at the
boiling temperature, 100 °C. The American
Meteorological Society, cited 2019: Saturation.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation American
Meteorological Society, cited 2019: Vapor pressure.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
152 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">vapor pressure value
es(100 °C)=101.325 kPa, is the same value as the
atmospheric surface pressure. Water boils at the
Earth’s surface when the American Meteorological
Society, cited 2019: Saturation. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation American
Meteorological Society, cited 2019: Vapor pressure.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
class="glossaryLink">vapor pressure is equal to the
atmospheric pressure, which is why water boils at
100 °C. Will water boil at the same temperature at the
top of Mount Everest?
Humidity Variables
American Meteorological Society, cited 2019: Vapor pressure.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
class="glossaryLink">Vapor pressure is one way of defining
humidity, but there are many others. Here is a noncomprehensive list of humidity variables and their typical units.
e = American Meteorological Society, cited 2019: Vapor
pressure. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
class="glossaryLink">vapor pressure (kPa)
r = mixing ratio (g·kg–1)
q = specific humidity (g·kg–1)
ATMO 200 • 153
ρv = absolute humidity (g·m-3)
RH = relative humidity (%)
zLCL = American Meteorological Society, cited 2019: Lifting
Condensation Level. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/
Lifting_condensation_level.]"
class="glossaryLink">lifting
condensation level (km)
Td = dew point (temperature) (°C)
Tw = wet-bulb temperature (°C)
American Meteorological Society, cited 2019: Vapor
pressure. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
class="glossaryLink">Vapor Pressure
We’ve already discussed American Meteorological Society,
cited 2019: Saturation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation American Meteorological
Society, cited 2019: Vapor pressure. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Vapor_pressure.]" class="glossaryLink">vapor pressure, es, but
you can also compute American Meteorological Society, cited
2019: Vapor pressure. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
class="glossaryLink">vapor pressure, e. However, because Td is
often unknown, the easiest way is usually through relative
humidity.
154 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Again, e0 is 0.6113 kPa, Lv is 2.5×106 J·kg–1, Rv is 461
J·K–1·kg–1, T0 is 273.15 K, and Td is dew point temperature,
which will be defined later.
Mixing Ratio
Mixing ratio, r, is the ratio of the mass of water vapor to the
mass of dry air. It is typically expressed as grams of water
vapor per kilogram of air (g·kg–1).
Pressure (P) should be in the same units as American
Meteorological Society, cited 2019: Vapor pressure. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Vapor_pressure.]" class="glossaryLink">vapor pressure
(e). The constant ε is 0.622, is the ratio between the gas constant
for dry air and the gas constant for water vapor.
American Meteorological Society, cited 2019: Saturation.
Glossary
of
Meteorology.
[Available
online
at
ATMO 200 • 155
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">Saturation mixing ratio, rs, is computed
the same way as the mixing ratio but with American
Meteorological Society, cited 2019: Saturation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Saturation.]" class="glossaryLink">saturation American
Meteorological Society, cited 2019: Vapor pressure. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Vapor_pressure.]" class="glossaryLink">vapor pressure,
es, instead of e.
When calculating mixing ratio, the pressure units on the top of
the fraction will cancel with the pressure units on the bottom of
the fraction. While it appears unit-less, its technically not based
on its definition of mass of water vapor as compared to mass of
dry air. See the Pro Tip below for more information.
Pro Tip: Many units of moisture are given in g·kg-1
or kg·kg-1 so technically the units could cancel and it
could be unitless! Don’t let this fool you. It is
important to remember that the mass in the numerator
and denominator are different. In the case of mixing
ratio, the value is given in mass of water vapor
proportional to the mass of dry air.
Specific Humidity
156 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Specific humidity, q, is the ratio of the mass of water vapor to
the total mass of air (dry air and water vapor combined). It is
expressed as grams of water vapor per kilogram of air (g·kg–1).
Again, American Meteorological Society, cited 2019:
Saturation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation specific humidity, qs, is
computed with es instead of e.
Absolute Humidity
Absolute humidity, ρv, is the ratio of the mass of water vapor to
the volume of air. It is expressed as grams of water vapor in a
cubic meter of air (g·m-3). It is effectively water vapor density.
Again,
American
Meteorological
Society,
cited
2019:
ATMO 200 • 157
Saturation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation absolute humidity, ρvs, uses es
instead of e.
Relative Humidity
Relative humidity, RH, is the ratio of the amount of water vapor
present in the air to the maximum amount of water vapor
needed for American Meteorological Society, cited 2019:
Saturation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation at a certain pressure and
temperature. It is typically multiplied by 100 and expressed as a
percent. Relative humidity shows how close the air is to being
saturated, not how much water vapor the air contains. For this
reason, RH is not a good indicator of the quantitative amount of
water vapor in the air. It is only a relative measure that is highly
dependent on the air temperature. Relative humidity greater
than 100% is called supersaturation.
or
158 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Imagine two parcels of air with the same volume,
pressure, and relative humidity. Parcel 1 has an air
temperature of 20 °C while Parcel 2 has an air
temperature of 30 °C. Which parcel contains more
water vapor?
Dew Point Temperature
The dew point temperature, Td, is the temperature to which the
air must be cooled to reach American Meteorological Society,
cited 2019: Saturation. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation, without changing the
moisture or air pressure. It measures the actual moisture content
of a parcel of air. American Meteorological Society, cited 2019:
Saturation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">Saturation occurs when the dew point
temperature equals the air temperature.
When the dew point temperature is lower than the freezing point
of water, it is also called the frost point.
Wet-Bulb Temperature
ATMO 200 • 159
Wet-bulb temperature, Tw, is the lowest temperature that can be
achieved if water evaporates within the air. When the relative
humidity is 100%, the wet-bulb temperature is equal to the air
temperature because there is no evaporation.
The wet-bulb temperature is difficult to calculate but easy to
measure. To measure the wet-bulb temperature, all you need is
a thermometer with a wet cloth wrapped around the bulb.
Typically this thermometer is attached to an apparatus called a
sling psychrometer to make it easy to spin around in the air to
create lots of airflow over the wet cloth on the thermometer.
The evaporation from the wet cloth cools the temperature
measured, hence the wet-bulb temperature is always lower than
the air temperature (or dry-bulb temperature) when relative
humidity is less than 100%.
You can also estimate the wet bulb temperature using lines
on a graph. Normand’s Rule is used to calculate the wet-bulb
temperature from the air temperature and the dew point
temperature. The wet-bulb temperature is always between the
dew point and the dry-bulb temperature (Td≤ Tw ≤ T). This
can be implemented on thermodynamic diagrams, such as the
Skew-T log P, which is discussed in more detail in the next
chapter.
Take note of this description for later. To find the wet-bulb
temperature on a Skew-T log P diagram, follow the American
Meteorological Society, cited 2019: Dry-adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate line upward from
160 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
the air temperature. Next, use the dew point temperature and
follow an isohume (line of constant relative humidity) upward.
The point where these two lines meet is called the American
Meteorological Society, cited 2019: Lifting Condensation Level.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Lifting_condensation_level.]"
class="glossaryLink">lifting condensation level (LCL). From
the meeting point, follow the moist (saturated) adiabatic
American Meteorological Society, cited 2019: Lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Lapse_rate.]"
class="glossaryLink">lapse rate back down to obtain the wetbulb temperature value. This is probably confusing at this point
because we have not discussed the LCL or the American
Meteorological Society, cited 2019: Moist adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Moist-adiabatic_lapse_rate.]"
class="glossaryLink">moist adiabatic lapse rate, but don’t
worry, we’ll repeat this logic again in the next chapter to make
sure this is clear.
Why Do We Care So Much About Moisture?
You may be wondering at this point why we care so much
about moisture and why we need so many definitions of (almost)
the same thing. The reason is that moisture is an extremely
important atmospheric property. Water can exist in three phases
(vapor, liquid, ice) within the atmosphere at typical pressures
and temperatures. It has an especially large impact on the human
experience—think about a humid day, foggy conditions, rain,
ATMO 200 • 161
snow, or even hail! Less obvious is its impact on atmospheric
American Meteorological Society, cited 2019: Stability.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability,
which
drives
the
aforementioned conditions.
For now, let’s think about the process of water vapor condensing
to form liquid water. There is one final definition of humidity
that will be helpful.
American Meteorological Society, cited 2019: Lifting
Condensation Level. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Lifting_condensation_level.]" class="glossaryLink">Lifting
Condensation Level
The American Meteorological Society, cited 2019: Lifting
Condensation Level. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/
Lifting_condensation_level.]" class="glossaryLink">lifting
condensation level, zLCL, is the altitude where clouds form. At
the LCL, temperature equals the dew point temperature,
resulting in American Meteorological Society, cited 2019:
Saturation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation and therefore condensation.
The height (z) of the LCL is
162 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
where a is 0.125 km °C-1. We can also define the temperature at
the LCL as follows.
Moist Adiabatic Lapse Rate
In the last chapter, we discussed how temperature changes as
a dry parcel of air is lifted in the atmosphere. You will recall
that as an American Meteorological Society, cited 2019: Air
parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel is lifted, the temperature drops
by 9.8 K every km due to the American Meteorological Society,
cited 2019: Work. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Work.]"
class="glossaryLink">work the American Meteorological
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel must do to the
environment as it expands. Let’s add moisture to the discussion
and see how this changes things.
If the American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air
parcel
reaches
American
Meteorological Society, cited 2019: Saturation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Saturation.]" class="glossaryLink">saturation (100%
relative humidity) and water vapor condenses to liquid water
ATMO 200 • 163
within the parcel, American Meteorological Society, cited 2019:
Latent heat. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Latent_heat.]"
class="glossaryLink">latent heat will be released. In the case
of a rising American Meteorological Society, cited 2019: Air
parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel that is cooling from adiabatic
expansion, this added heat from condensation counterbalances
some of the cooling. Hence, the American Meteorological
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel will no longer
cool at the American Meteorological Society, cited 2019: Dryadiabatic lapse rate. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate but at the smaller
American Meteorological Society, cited 2019: Moist adiabatic
lapse rate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Moist-adiabatic_lapse_rate.]"
class="glossaryLink">moist adiabatic lapse rate (Γm). Unlike
the dry adiabatic lapse, the American Meteorological Society,
cited 2019: Moist adiabatic lapse rate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Moistadiabatic_lapse_rate.]" class="glossaryLink">moist adiabatic
lapse rate is not constant and varies based on the temperature and
moisture of the American Meteorological Society, cited 2019:
Air parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel.
164 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
We will approximate the American Meteorological Society,
cited 2019: Moist adiabatic lapse rate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Moistadiabatic_lapse_rate.]" class="glossaryLink">moist adiabatic
lapse rate with the following value.
The difference between the American Meteorological Society,
cited 2019: Dry-adiabatic lapse rate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Dryadiabatic_lapse_rate.]" class="glossaryLink">dry adiabatic
lapse rate (Γm) and the American Meteorological Society, cited
2019: Moist adiabatic lapse rate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Moistadiabatic_lapse_rate.]" class="glossaryLink">moist adiabatic
lapse rate (Γm) is significant and has profound influence on
atmospheric American Meteorological Society, cited 2019:
Stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability, the topic of the following
chapter.
Chapter 4: Questions to Consider
1.
Explain the conditions needed for
American Meteorological Society, cited
ATMO 200 • 165
2019: Saturation. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Saturation.]"
class="glossaryLink">saturation to
occur.
2.
What is the American Meteorological
Society, cited 2019: Saturation. Glossary
of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Saturation.]"
class="glossaryLink">saturation
American Meteorological Society, cited
2019: Vapor pressure. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Vapor_pressure.]"
class="glossaryLink">vapor pressure of
air at 26°C?
3.
Explain the difference between
specific humidity and relative humidity.
4.
If the temperature is 10°C and the
pressure is 700 hPa, calculate the
American Meteorological Society, cited
2019: Saturation. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Saturation.]"
class="glossaryLink">saturation specific
humidity and the American
166 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorological Society, cited 2019:
Saturation. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Saturation.]"
class="glossaryLink">saturation mixing
ratio.
5.
Explain why the American
Meteorological Society, cited 2019:
Moist adiabatic lapse rate. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Moistadiabatic_lapse_rate.]"
class="glossaryLink">moist adiabatic
lapse rate is less than the American
Meteorological Society, cited 2019: Dryadiabatic lapse rate. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Dryadiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse
rate.
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=107
Chapter 5: Atmospheric Stability
ALISON NUGENT AND DAVID DECOU
Learning Objectives
By the end of this chapter, you should be able to:
1.
Interpret American Meteorological
Society, cited 2019: Stability. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Stability.]"
class="glossaryLink">stability based on
the dry and moist adiabatic lapse rates
2.
Understand how American
Meteorological Society, cited 2019:
Stability. Glossary of Meteorology.
167
168 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
[Available online at
http://glossary.ametsoc.org/wiki/
Stability.]"
class="glossaryLink">stability relates to
vertical motion in the atmosphere
3.
Describe and differentiate between the
many lines on a Skew-T log-P diagram
4.
Find the LCL, regions of CAPE and
CIN, and the tropopause from a Skew-T
log-P diagram
A cumulonimbus cloud rises and expands as a product of instability
within the atmosphere (Public Domain).
ATMO 200 • 169
Introduction
When you think of the word “stable,” you typically think of an
object that is unlikely to change or something that is balanced.
The opposite is true with something that is “unstable”. An
unstable object is likely to fall or change position with time. The
same is true with clouds. When you see a fluffy cumulus cloud,
you might notice them changing shape from one minute to the
next. Such clouds are in a constant state of change, and thus
represent the atmosphere in an unstable state.
A perfect cumulus cloud just west of Magic Island (photo by Sarah
Williamson).
American Meteorological Society, cited 2020: Instability.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Instability.]"
class="glossaryLink">Instability in the atmosphere is a concept
170 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
that is intimately connected with thunderstorms, cumulus
development, and vertical motion. In order to visualize the
concept of American Meteorological Society, cited 2019:
Stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability, you might imagine a boulder
sitting at the bottom of a canyon surrounded by steep hills, as
depicted in the figure below by the blue circle. If you were
strong enough to push the boulder from its initial position
partway up one of the hills, it would roll back to the bottom
once you let go. Despite exerting a force on the boulder and
causing an initial displacement, it would return to its initial
position, and the net displacement would be zero. To visualize
the concept of American Meteorological Society, cited 2020:
Instability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Instability.]"
class="glossaryLink">instability, imagine the same boulder at
the top of a hill (red circle below). If you were able to push
the boulder just a little bit in any direction, it would begin to
roll downward and accelerate away from its initial position.
However, if the same boulder were to be placed on flat ground
(green circle below) and you were to push it, it would change
position, but remain in its new position. This is an example
of American Meteorological Society, cited 2019: Neutral
stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Neutral_stability.]"
class="glossaryLink">neutral stability.
Each of these concepts can be applied to motions of air parcels in
the atmosphere. The topic of American Meteorological Society,
ATMO 200 • 171
cited 2019: Stability. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability in atmospheric science is
important because the formation of clouds is closely related
to American Meteorological Society, cited 2019: Stability.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability or American Meteorological
Society, cited 2020: Instability. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Instability.]"
class="glossaryLink">instability
in
the
atmosphere. In this chapter we will connect these concepts to
the buoyancy of air parcels, and learn to use thermodynamic
diagrams to visualize movement.
172 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Examples of stability and instability in relation to air and parcel
temperatures (created by Britt Seifert).
Atmospheric Stability & Lapse Rates
American Meteorological Society, cited 2019: Adiabatic
ATMO 200 • 173
processes. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Adiabatic_processes.]"
class="glossaryLink">Adiabatic Processes
When discussing American Meteorological Society, cited 2019:
Stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability in atmospheric sciences, we
typically think about air parcels, or imaginary blobs of air that
can expand and contract freely, but do not mix with the air
around them or break apart. The key piece of information is that
movement of air parcels in the atmosphere can be estimated as
an adiabatic process. American Meteorological Society, cited
2019: Adiabatic processes. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Adiabatic_processes.]"
class="glossaryLink">Adiabatic
processes do not exchange heat and they are reversible.
Imagine you have a parcel of air at the Earth’s surface. The
American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel has the same temperature and
pressure as the surrounding air, which we will call the
environment. If you were to lift the American Meteorological
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel, it would find
itself in a place where the surrounding environmental air
pressure is lower, because we know that pressure decreases with
174 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
height. Because the environmental air pressure outside the parcel
is lower than the pressure inside the parcel, the air molecules
inside the parcel will effectively push outward on the walls of
the parcel and expand adiabatically. The air molecules inside the
parcel must use some of their own American Meteorological
Society, cited 2019: Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy in order to expand the American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel’s walls, so
the temperature inside the parcel decreases as the internal
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy decreases. To
summarize, rising air parcels expand and cool adiabatically
without exchanging heat with the environment.
Now imagine that you move the same American Meteorological
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel back to Earth’s
surface. The American Meteorological Society, cited 2019: Air
parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel is moving into an environment
with higher air pressure. The higher environmental pressure will
push inward on the parcel walls, causing them to compress, and
raise the inside temperature.
ATMO 200 • 175
The process is adiabatic, so again, no heat is exchanged with the
environment. However, temperature changes in the American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel can still
occur, but it is not due to mixing, it is due to changes in the
internal American Meteorological Society, cited 2019: Energy.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy of the American Meteorological
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel.
American Meteorological Society, cited 2019: Dry-adiabatic
lapse rate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Dryadiabatic_lapse_rate.]"
class="glossaryLink">Dry
Adiabatic Lapse Rate
As long as an American Meteorological Society, cited 2019:
Air parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel is unsaturated (relative
humidity < 100%), the rate at which its temperature will change
will be constant. A decrease in temperature with height is called
a American Meteorological Society, cited 2019: Lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Lapse_rate.]"
class="glossaryLink">lapse rate and while the temperature
176 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
decreases with altitude, it is defined as positive because it is
a lapse rate. Recall from chapter 3 that the American
Meteorological Society, cited 2019: Dry-adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Dryadiabatic_lapse_rate.]" class="glossaryLink">dry adiabatic
lapse rate, Γd, is equal to 9.8 K·km-1 = 9.8 °C·km-1. This drop
in temperature is due to adiabatic expansion and a decrease in
internal American Meteorological Society, cited 2019: Energy.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy.
Air rises, expands, and cools at the dry adiabatic lapse rate,
approximated as a 10°C decrease per km (created by Britt Seifert).
ATMO 200 • 177
Let’s get back to the topic of atmospheric American
Meteorological Society, cited 2019: Stability. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Stability.]"
class="glossaryLink">stability. American
Meteorological Society, cited 2019: Stability. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Stability.]"
class="glossaryLink">Stability
in
the
atmosphere refers to a condition of equilibrium. As discussed
with the example of the boulder on a hill or valley, some initial
movement resulted in either more (unstable), less (stable), or
no change (neutral). Given some initial change in the elevation
of an American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel, if the air is in stable
equilibrium, the parcel will tend to return back to its original
position after it is forced to rise or sink. In an unstable
equilibrium, an American Meteorological Society, cited 2019:
Air parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel will accelerate away from its
initial position after being pushed. The motion could be upward
or downward, but generally unstable atmospheres favors vertical
motions. Finally, in a neutral equilibrium, some initial change
in the elevation of an American Meteorological Society, cited
2019: Air parcel. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
178 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">air parcel will not result in any additional
movement.
Determining American Meteorological Society, cited 2019:
Stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">Stability
How do you know if an American Meteorological Society, cited
2019: Air parcel. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel will be stable after some initial
displacement? American Meteorological Society, cited 2019:
Stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">Stability is determined by comparing the
temperature of a rising or sinking American Meteorological
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel to the
environmental air temperature. Imagine the following: at some
initial time, an American Meteorological Society, cited 2019:
Air parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel has the same temperature and
pressure as its environment. If you lift the American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel some
distance, its temperature drops by 9.8 K·km-1, which is the
ATMO 200 • 179
American Meteorological Society, cited 2019: Dry-adiabatic
lapse rate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate. If the American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel is colder
than the environment in its new position, it will have higher
density and tend to sink back to its original position. In this
case, the air is stable because vertical motion is resisted. If the
rising air is warmer and less dense than the surrounding air,
it will continue to rise until it reaches some new equilibrium
where its temperature matches the environmental temperature.
In this case, because an initial change is amplified, the American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel is unstable.
In order to figure out if the American Meteorological Society,
cited 2019: Air parcel. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel is unstable or not we must
know the temperature of both the rising air and the environment
at different altitudes.
One way this is done in practice is with a American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather balloon. We can
get a vertical profile of the environmental American
Meteorological Society, cited 2019: Lapse rate. Glossary of
180 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Lapse_rate.]" class="glossaryLink">lapse rate by releasing
a American Meteorological Society, cited 2020: Radiosonde.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiosonde.]"
class="glossaryLink">radiosonde attached to a American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather balloon. A
American Meteorological Society, cited 2020: Radiosonde.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiosonde.]"
class="glossaryLink">radiosonde sends back data on
temperature, humidity, wind, and position, which are plotted on
a thermodynamic diagram. This vertical plot of temperature and
other variables is known as a sounding.
Dry American Meteorological Society, cited 2019: Stability.
Glossary
of
Meteorology.
[Available
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">Stability
online
at
If an American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel is dry, meaning unsaturated,
American Meteorological Society, cited 2019: Stability.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability is relatively straightforward. An
ATMO 200 • 181
atmosphere where the environmental American Meteorological
Society, cited 2019: Lapse rate. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Lapse_rate.]" class="glossaryLink">lapse rate is the same as
the American Meteorological Society, cited 2019: Dry-adiabatic
lapse rate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate, meaning that the
temperature in the environment also drops by 9.8 K·km-1, will
be considered neutrally stable. After some initial vertical
displacement, the temperature of the American Meteorological
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel will always be the
same as the environment so no further change in position is
expected.
If the environmental American Meteorological Society, cited
2019: Lapse rate. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Lapse_rate.]"
class="glossaryLink">lapse rate is less than the American
Meteorological Society, cited 2019: Dry-adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate, some initial
vertical displacement of the American Meteorological Society,
cited 2019: Air parcel. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel will result in the American
Meteorological Society, cited 2019: Air parcel. Glossary of
182 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel either being
colder than the environment (if lifted), or warmer than the
environment (if pushed downward). This is because if lifted,
the temperature of the American Meteorological Society, cited
2019: Air parcel. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel would drop more than the
temperature of the environment. This is a stable situation for a
dry American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel and a typical scenario in the
atmosphere. The global average tropospheric American
Meteorological Society, cited 2019: Lapse rate. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Lapse_rate.]"
class="glossaryLink">lapse rate is 6.5 K·km-1, which is
stable for dry lifting.
Finally, if the environmental American Meteorological Society,
cited 2019: Lapse rate. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Lapse_rate.]"
class="glossaryLink">lapse rate is greater than the American
Meteorological Society, cited 2019: Dry-adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate, some initial
vertical displacement of the American Meteorological Society,
cited 2019: Air parcel. Glossary of Meteorology. [Available
ATMO 200 • 183
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel will result in the American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel either being
warmer than the environment (if lifted), or colder than the
environment (if pushed downward). This is because if lifted,
the temperature of the American Meteorological Society, cited
2019: Air parcel. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel would drop less than the
temperature of the environment. This is an unstable situation
for a dry American Meteorological Society, cited 2019: Air
parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel.
In general for a dry American Meteorological Society, cited
2019: Air parcel. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel, the following is true.
UNSTABLE
\end{align*}"
QuickLaTeX.com">
\Gamma_{env} \:\:\:\:\:
title="Rendered
by
American Meteorological Society, cited 2019: Moist
adiabatic lapse rate. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Moist-
184 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
adiabatic_lapse_rate.]"
Adiabatic Lapse Rate
class="glossaryLink">Moist
When moisture is added, everything gets more complicated.
In Chapter 4 we learned that whether or not an American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel is saturated
depends primarily on its temperature and, of course, its moisture
content. The graph of the Clausius-Clapeyron relationship
shows us that given the same amount of moisture, air is more
likely to be saturated at a lower temperature.
We know that as an American Meteorological Society, cited
2019: Air parcel. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel is lifted, its temperature drops
according to the American Meteorological Society, cited 2019:
Dry-adiabatic lapse rate. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Dryadiabatic_lapse_rate.]" class="glossaryLink">dry adiabatic
lapse rate. So what happens when the American Meteorological
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel is cold enough
that the air becomes saturated with respect to water vapor? The
short answer is that if it continues to cool, water vapor will
condense to liquid water to form a cloud.
When water vapor condenses, it goes from a higher American
Meteorological Society, cited 2019: Energy. Glossary of
ATMO 200 • 185
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy state to a lower
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy state. American
Meteorological Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">Energy is never created
nor destroyed, especially in phase changes, so what happens to
all that excess American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy? The American Meteorological
Society, cited 2019: Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy gets released in the form of
American Meteorological Society, cited 2019: Latent heat.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Latent_heat.]"
class="glossaryLink">latent heat. The American Meteorological
Society, cited 2019: Latent heat. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Latent_heat.]" class="glossaryLink">latent heat of condensation
is approximately equal to 2.5 * 106 J·kg-1, which means that for
every kg of water vapor that condenses to form liquid water, 2.5
*106 Joules of American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy are released.
186 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
This has large consequences for the American Meteorological
Society, cited 2019: Lapse rate. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Lapse_rate.]" class="glossaryLink">lapse rate of an American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel and
distinguishes the American Meteorological Society, cited 2019:
Dry-adiabatic lapse rate. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Dryadiabatic_lapse_rate.]" class="glossaryLink">dry adiabatic
lapse rate from the American Meteorological Society, cited
2019: Moist adiabatic lapse rate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Moistadiabatic_lapse_rate.]" class="glossaryLink">moist adiabatic
lapse rate. As American Meteorological Society, cited 2019:
Latent heat. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Latent_heat.]"
class="glossaryLink">latent heat is added from the process of
condensation, it offsets some of the adiabatic cooling from
expansion. Because of this, the American Meteorological
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel will no longer
cool at the American Meteorological Society, cited 2019: Dryadiabatic lapse rate. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate, but will cool as a
slower rate, known as the American Meteorological Society,
cited 2019: Moist adiabatic lapse rate. Glossary of
ATMO 200 • 187
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Moistadiabatic_lapse_rate.]"
class="glossaryLink">moist
adiabatic lapse rate. To summarize, a parcel will cool at the dry
adiabatic rate until it is saturated, after which it won’t cool
as quickly due to condensation. The American Meteorological
Society, cited 2019: Moist adiabatic lapse rate. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Moist-adiabatic_lapse_rate.]" class="glossaryLink">moist
adiabatic lapse rate varies a little by temperature, but in this class
we will consider it a constant for simplicity: Γm = 4.5 K·km-1 =
4.5 °C·km-1
Moist American Meteorological Society, cited 2019:
Stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">Stability
The effects of moisture change the American Meteorological
Society, cited 2019: Lapse rate. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Lapse_rate.]" class="glossaryLink">lapse rate of the American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel and,
therefore, affects American Meteorological Society, cited 2019:
Stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability. However, the concepts are still
the same and we still compare the American Meteorological
188 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel temperature to the
environmental temperature. We have just one added
complication to worry about—we need to know whether the
American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel is dry or moist. Some
definitions are included below, which take into account both dry
and moist adiabatic lapse rates.
ATMO 200 • 189
A thermodynamic diagram showing the stability of the atmosphere
based on the dry (Γd = 9.8 K km-1) and moist (Γm = 4.5 K km-1)
adiabatic lapse rates (Created by Britt Seifert).
The atmosphere is said to be absolutely stable if the
environmental American Meteorological Society, cited 2019:
Lapse rate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Lapse_rate.]"
class="glossaryLink">lapse rate is less than the American
Meteorological Society, cited 2019: Moist adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Moist-adiabatic_lapse_rate.]"
class="glossaryLink">moist adiabatic lapse rate. This means
that a rising American Meteorological Society, cited 2019: Air
190 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel will always cool at a faster rate
than the environment, even after it reaches American
Meteorological Society, cited 2019: Saturation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Saturation.]" class="glossaryLink">saturation. If an
American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel is cooler at all levels, then it
will not be able to rise, even after it becomes saturated (when
latent heating will counteract some cooling).
The atmosphere is said to be absolutely unstable if the
environmental American Meteorological Society, cited 2019:
Lapse rate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Lapse_rate.]"
class="glossaryLink">lapse rate is greater than the American
Meteorological Society, cited 2019: Dry-adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate. This means that a
rising American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel will always cool at a slower rate
than the environment, even when it is unsaturated. This means
that it will be warmer (and less dense) than the environment, and
allowed to rise.
ATMO 200 • 191
The atmosphere is said to be conditionally unstable if the
environmental American Meteorological Society, cited 2019:
Lapse rate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Lapse_rate.]"
class="glossaryLink">lapse rate is between the moist and dry
adiabatic lapse rates. This means that the buoyancy (the ability
of an American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel to rise) of an American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel depends on
whether or not it is saturated. In a conditionally unstable
atmosphere, an American Meteorological Society, cited 2019:
Air parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel will resist vertical motion when
it is unsaturated, because it will cool faster than the environment
at the American Meteorological Society, cited 2019: Dryadiabatic lapse rate. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate. If it is forced to
rise and is able to become saturated, however, it will cool at the
American Meteorological Society, cited 2019: Moist adiabatic
lapse rate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Moist-adiabatic_lapse_rate.]"
class="glossaryLink">moist adiabatic lapse rate. In this case, it
will cool slower than the environment, become warmer than the
environment, and will rise.
192 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Hawaiian Focus Box
Around Hawaii, the atmosphere is almost always
conditionally unstable, meaning that the
environmental American Meteorological Society,
cited 2019: Lapse rate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Lapse_rate.]" class="glossaryLink">lapse rate lies
somewhere between the dry and moist adiabatic
lapse rates. For this reason, Hawaii almost always
has convective clouds. Convective clouds are clouds
where the edges are bumpy and American
Meteorological Society, cited 2019: Cumuliform.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Cumuliform.]"
class="glossaryLink">cumuliform, like cauliflower.
The clouds are convective because the atmosphere is
stable to dry lifting and unstable to moist lifting.
Once the air is saturated, American Meteorological
Society, cited 2020: Instability. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Instability.]"
class="glossaryLink">instability sets in and vertical
motion takes off. This is especially common as air is
lifted over our mountainous islands. The forced
lifting from the terrain creates clouds and rain right
over the mountains! In scientific terms, the initial
lifting of the stable low level dry air by the terrain
causes the air to adiabatically expand and reach
American Meteorological Society, cited 2019:
Saturation. Glossary of Meteorology. [Available
ATMO 200 • 193
online at http://glossary.ametsoc.org/wiki/
Saturation.]" class="glossaryLink">saturation, at
which point the environment is unstable to moist
lifting and convection is the result.
Skew-T Log-P Diagram
There are many different types of thermodynamic diagrams,
but the main one we will discuss are Skew-T Log-P diagrams,
so-named because the American Meteorological Society, cited
2020: Isotherms. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Isotherms.]"
class="glossaryLink">isotherms (lines of equal temperature, T)
on the diagram are slanted (skewed) and the American
Meteorological Society, cited 2020: Isobars. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Isobars.]" class="glossaryLink">isobars (lines of equal
pressure, P) on the diagram are in log space. Here we will
focus on how to read and utilize Skew-T Log-P diagrams (often
shortened to Skew-T diagram) to determine parcel buoyancy
and atmospheric American Meteorological Society, cited 2019:
Stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability.
194 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
An example Skew-T Log-P diagram from Lihue on August 31st, 2018.
The sounding was retrieved from the Upper Air Soundings portion of
the University of Wyoming Weather Web: http://weather.uwyo.edu/
upperair/sounding.html (Copyright 2018 by University
of Wyoming Department of Atmospheric Science, used with
permission.)
The American Meteorological Society, cited 2020: Radiosonde.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiosonde.]"
class="glossaryLink">radiosonde balloon sounding plotted here
was launched from Lihue on Kauai (see the top left, labelled
as station “91165 PHLI Lihue”). You can see the vertical
environmental temperature profile (T) plotted as the black
jagged line on the right. The dew point temperature (Td) with
height is plotted with the black jagged line on the left. Although
this figure may be overwhelming to read at first, we’ll walk
ATMO 200 • 195
through it together. The horizontal axis is temperature in °C,
with temperatures increasing to the right. The vertical axis is
air pressure in hPa, decreasing with height, so higher heights
are toward the top of the chart. When the T and Td lines are
close together, the environment has a high relative humidity
and the air is closer to American Meteorological Society, cited
2019: Saturation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation. In this particular sounding,
there is a lot of moisture near the surface, but dries out in the
mid-levels.
American Meteorological Society, cited 2020: Radiosonde.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiosonde.]"
class="glossaryLink">Radiosonde balloons are launched twice
a day (00Z and 12Z) from many locations around the world. The
latitude and longitude for the station is given in the top of the list
on the right where station latitude (SLAT) is given as 21.99
degrees North and SLON is -159.34 degrees West. The station
elevation SELV is 30 m. The sounding time and date is given
in the bottom left, and the bottom right says “University of
Wyoming” because in this particular example, the University of
Wyoming is the organization that gathered and archived the
dataset. You can find soundings for other locations and dates at
this website: http://weather.uwyo.edu/upperair/sounding.html.
Let’s go through the lines one by one.
196 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Isobars (horizontal, lines of constant pressure) and isotherms (slanted,
lines of constant temperature) (CC BY-NC-SA 4.0).
The horizontal lines on a Skew-T are American Meteorological
Society, cited 2020: Isobars. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars, or lines of equal air pressure. You
will typically see them given in hPa, but the lines in the above
figure are in kPa. The American Meteorological Society, cited
2020: Isobars. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars have larger spaces as you get
toward the top of the diagram because they are logarithmic
with height. The evenly-spaced solid lines that slant up and
to the right are American Meteorological Society, cited 2020:
Isotherms. Glossary of Meteorology. [Available online at
ATMO 200 • 197
http://glossary.ametsoc.org/wiki/Isotherms.]"
class="glossaryLink">isotherms, or lines of equal temperature
(T). This allows colder temperatures to be plotted on the
diagram.
Isohumes (slanted dashed lines), lines of constant mixing ratio (CC
BY-NC-SA 4.0).
The dashed lines that run up and to the right are isohumes,
or lines of constant mixing ratio. These are typically given in
units of g·kg–1. If you use a Skew-T where these lines are not
dashed or color-coded, remember that these are spaced more
closely together than American Meteorological Society, cited
2020: Isotherms. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Isotherms.]"
198 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">isotherms and are more steep. They also
do not line up with the temperature labels on the x-axis.
Dry adiabatic lapse rate reference lines, also known as lines of constant
potential temperature (CC BY-NC-SA 4.0).
The evenly-spaced curved solid lines that run from bottom right
to top left are dry adiabats, and depict the American
Meteorological Society, cited 2019: Dry-adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate (9.8 K·km-1). The
American Meteorological Society, cited 2019: Dry-adiabatic
lapse rate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate is considered a
ATMO 200 • 199
constant, but you can see here that over large changes in
temperature and pressure, it varies a little. Don’t worry about
these variations—we still consider it a constant. American
Meteorological Society, cited 2019: Dry-adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">Dry adiabatic lapse rate reference lines
are also called lines of constant American Meteorological
Society, cited 2019: Potential temperature. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Potential_temperature.]" class="glossaryLink">potential
temperature (θ). The dry adiabats always curve upward from
right to left in a concave way.
Moist adiabatic lapse rate reference lines. (CC BY-NC-SA 4.0).
200 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
The uneven, dashed, lines that curve up and to the left are
the moist adiabats. The American Meteorological Society, cited
2019: Moist adiabatic lapse rate. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Moistadiabatic_lapse_rate.]" class="glossaryLink">moist adiabatic
lapse rate varies with both temperature and moisture content,
but is close to the American Meteorological Society, cited 2019:
Dry-adiabatic lapse rate. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Dryadiabatic_lapse_rate.]" class="glossaryLink">dry adiabatic
lapse rate at high altitudes due to cold temperatures and small
moisture content. These lines are parallel to the dry adiabats
higher up on the Skew-T Log-P diagram. These are also lines
of constant equivalent American Meteorological Society, cited
2019: Potential temperature. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Potential_temperature.]"
class="glossaryLink">potential
temperature (θe).
ATMO 200 • 201
A complete Skew-T Log-P diagram, used to visualize changes in the
atmosphere with altitude. (CC BY-NC-SA 4.0).
Here is a complete Skew-T Log-P diagram. All of the lines look
confusing and complicated when combined, but each represents
a constant change in one variable.
Let’s look at another real balloon sounding. This time launched
from Hilo during Hurricane Lane.
202 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Balloon sounding launched from Hilo as Hurricane Lane impacted the
Big Island. The sounding was retrieved from the Upper Air Soundings
portion of the University of Wyoming Weather
Web: http://weather.uwyo.edu/upperair/sounding.html. (Copyright 2018
by University of Wyoming Department of Atmospheric Science, used
with permission.)
On this Skew-T diagram, all of the same lines are there.
Horizontal blue lines are American Meteorological Society,
cited 2020: Isobars. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars, slanted blue lines are American
Meteorological Society, cited 2020: Isotherms. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Isotherms.]" class="glossaryLink">isotherms, slanted
purple lines are isohumes, the green lines are the dry adiabats,
and the blue curved lines are the moist adiabats. The T (right)
ATMO 200 • 203
and Td (left) black lines are close together and sometimes
overlap in the lowest 500 hPa of the atmosphere because the
lower levels are incredibly moist, and a deep cloud layer
extended up to nearly 6 km altitude.
Finding the American Meteorological Society, cited 2019:
Lifting Condensation Level. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Lifting_condensation_level.]" class="glossaryLink">Lifting
Condensation Level (LCL)
When plotting a sounding on a Skew-T diagram, you may have
a selection of data similar to the example given below. You
will likely have pressure, temperature (T), and a dew point
temperature (Td) with altitude.
Sample atmospheric data to be plotted on the skew-T diagrams (CC
BY-NC-SA 4.0).
In order to plot the sounding, it is easiest to start by finding the
pressure level and then move to the right to plot the temperature
204 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
and dew point temperature. Pay careful attention to the fact that
the American Meteorological Society, cited 2020: Isotherms.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Isotherms.]"
class="glossaryLink">isotherms are skewed. Rotate the axis in
your mind when you plot your temperature and dew point. Once
you have plotted all of your temperatures and dew points, you
will have a vertical temperature and humidity profile of the
atmosphere.
Sample example plotted (CC BY-NC-SA 4.0).
Now that we plotted the sounding, it is useful to know how a
rising American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
ATMO 200 • 205
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel will behave when placed in this
environment. Is the atmosphere stable, unstable, or conditionally
unstable? We can determine this by estimating the rate at which
a rising parcel will cool and drawing a parcel path upward. A
rising American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel will cool at the American
Meteorological Society, cited 2019: Dry-adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate until it is
saturated, after which it will cool at the American
Meteorological Society, cited 2019: Moist adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Moist-adiabatic_lapse_rate.]"
class="glossaryLink">moist adiabatic lapse rate. How do we
know when a parcel will be saturated? First we need to find
the American Meteorological Society, cited 2019: Lifting
Condensation Level. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Lifting_condensation_level.]" class="glossaryLink">Lifting
Condensation Level (LCL).
The American Meteorological Society, cited 2019: Lifting
Condensation Level. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Lifting_condensation_level.]" class="glossaryLink">Lifting
Condensation Level (LCL) is the level at which the water
206 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
vapor in an American Meteorological Society, cited 2019: Air
parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel that is lifted dry adiabatically
will be saturated.
The red dot is air temperature and the blue circle is dew point
temperature. This diagram is an example of of an unsaturated air parcel.
Stull Figure 5.7 (CC BY-NC-SA 4.0).
To find the LCL, start at the surface (or the pressure level closest
to the surface, typically 1000 hPa) and plot the temperature and
dewpoint temperature. In the case of the example above, the
surface pressure level must be at a raised elevation with Psurf =
90 kPa or 900 hPa, T = 30 °C, and Td = -10 °C. Imagine that
the American Meteorological Society, cited 2019: Air parcel.
ATMO 200 • 207
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel has the same temperature and
dewpoint temperature as the environment at first. Initially, it will
cool at the American Meteorological Society, cited 2019: Dryadiabatic lapse rate. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Dry-adiabatic_lapse_rate.]"
class="glossaryLink">dry adiabatic lapse rate as it rises. First,
follow the surface temperature upward along a dry adiabat. In all
likelihood, the temperature will not be directly along a marked
dry adiabat line as it is in the example so follow a line upward
parallel to a dry adiabat. Similarly, start at your surface dew
point and follow the isohume (constant mixing ratio line)
upward because the moisture content of the American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel does not
change with dry lifting. Draw these lines upward until they
intersect. This intersection will give you the level of the
American Meteorological Society, cited 2019: Lifting
Condensation Level. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/
Lifting_condensation_level.]"
class="glossaryLink">lifting
condensation level (LCL).
208 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Follow the dry adiabat and isohume lines until they intersect (CC
BY-NC-SA 4.0).
ATMO 200 • 209
The place where the two lines intersect is the lifting condensation level
(CC BY-NC-SA 4.0).
In this example, the surface temperature and dewpoint
temperature line up nicely with an isohume and a dry adiabat
line, but this typically won’t be the case with a real sounding.
The procedure, however, will be the same. The LCL marks the
approximate cloud base height for convective clouds (cumulus
type), where rising air first becomes saturated.
After the American Meteorological Society, cited 2019: Air
parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel has been lifted dry adiabatically
to the LCL, it becomes saturated. As we know, a saturated
American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
210 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel cools at the smaller American
Meteorological Society, cited 2019: Moist adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Moist-adiabatic_lapse_rate.]"
class="glossaryLink">moist adiabatic lapse rate. From the
LCL, follow a line parallel to a moist adiabat upward to get
the approximate American Meteorological Society, cited 2019:
Lapse rate. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Lapse_rate.]"
class="glossaryLink">lapse rate of your parcel as it rises. In the
example soundings from Hilo and Lihue shown earlier, this same
line is plotted in a light grey color from the surface all the way up
in the atmosphere. It shows the temperature a surface based
parcel would have when lifted through the troposphere.
As you follow an American Meteorological Society, cited 2019:
Air parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel temperature upward moist
adiabatically, the point at which it intersects the environmental
temperature profile (where your parcel becomes warmer than its
environment) is called the American Meteorological Society,
cited 2019: Level of Free Convection. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Level_of_free_convection.]" class="glossaryLink">Level of
Free Convection, or the LFC.
As you continue following the American Meteorological
ATMO 200 • 211
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel path upward
moist adiabatically from the LFC, the point where it intersects
the sounding again (the point where your parcel becomes cooler
than its environment) is called the American Meteorological
Society, cited 2019: Level of neutral buoyancy. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Level_of_neutral_buoyancy.]"
class="glossaryLink">Equilibrium Level (EL).
Normand’s Rule for Wet-bulb Temperature
You can estimate the surface wet bulb temperature by taking
the LCL example one step further. Normand’s Rule is used to
calculate the wet-bulb temperature from the air temperature and
the dew point temperature. The wet bulb temperature is always
between the dew point and the dry bulb temperature (Td ≤
Tw ≤ T). To find the wet bulb temperature on a
Skew-T Log-P diagram, follow the surface T upwards along a
dry adiabat, and the surface Td upwards along a isohume. Where
they meet is the LCL, as just explained. Next, follow a moist
adiabat back down to the surface. Where the moist adiabat
intersects the surface is the wet-bulb temperature value.
212 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
CAPE & CIN
The “positive area” between the parcel path and the
environmental temperature profile, traced out between the LFC
and the EL (where the parcel is warmer than the environment)
gives a measure of the American Meteorological Society, cited
2019: Convective Available Potential Energy. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Convective_available_potential_energy.]"
class="glossaryLink">Convective
Available
Potential
–1
Energy, or CAPE, given in units of J·kg . This is an estimate
of the buoyant American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy of a parcel and can provide a
means of estimating the strength of any convection that may
occur. CAPE can also provide an estimate of the maximum
updraft intensity in a American Meteorological Society, cited
2020: Thunderstorm. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm.
wmax is the estimated maximum vertical motion as a result of
CAPE.
American Meteorological Society, cited 2019: Convective
Inhibition. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Convective_inhibition.]"
ATMO 200 • 213
class="glossaryLink">Convective Inhibition, or CIN is
essentially negative CAPE, also in J·kg–1. It is the negative
area between the parcel path and the environmental temperature
curve where the parcel is cooler than the environment. The
larger the value of CIN, the greater the negative buoyant
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy that acts against
CAPE. CIN sometimes acts as a “cap” on convection. If you
have large CAPE but also large CIN, your CAPE may not be
fully realized as buoyant American Meteorological Society,
cited 2019: Energy. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy and you may not have any
convection. However, if your parcel is able to break through the
cap, that is, if it is able to rise and become warmer than the
environment, convection may be strong.
The figure below shows the locations of the LFC and EL, and
shades in both positive and negative areas between the parcel
path and the environmental temperature profile.
214 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
The locations of the LFC and EL in the vertical sounding are shown,
found as the positive and negative areas between the parcel path and the
environmental temperature profile (Public Domain).
In the Lihue and Hilo soundings shown previously, values of
CAPE and CIN are given in J·kg–1 in the column on the right
hand side. Note that CIN is written as “CINS” and denoted as a
negative value.
ATMO 200 • 215
Locating the Tropopause
Recall that the standard temperature decreases with height
within the troposphere, but becomes isothermal with height
within the the tropopause, and increases with height in the
stratosphere. With this knowledge, the location of the
tropopause, given by its pressure level, can be determined by
examining a plotted sounding. In the upper part of your
sounding, look for where the temperature profile becomes
isothermal (parallel to your skewed American Meteorological
Society, cited 2020: Isotherms. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Isotherms.]" class="glossaryLink">isotherms) or for an
inversion (where the temperature increases with height, which
will be tilted to the right more than your American
Meteorological Society, cited 2020: Isotherms. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Isotherms.]" class="glossaryLink">isotherms). The base of
the isothermal layer in your sounding is the tropopause.
216 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
A plotted sounding with two isothermal layers and a temperature
inversion denoted (CC BY-NC-SA 4.0).
There are many things we can learn about the atmosphere from
Skew-T Log-P diagrams. Here we’ve provided just the basics to
get you started.
Chapter 5: Questions to Consider
1.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
ATMO 200 • 217
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=662
2.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=662
3.
Drag and drop terms to their correct
position in the atmospheric American
Meteorological Society, cited 2019:
Stability. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Stability.]"
class="glossaryLink">stability diagram
below:
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=662
4.
What does the American
Meteorological Society, cited 2019:
Lifting Condensation Level. Glossary of
Meteorology. [Available online at
218 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://glossary.ametsoc.org/wiki/
Lifting_condensation_level.]"
class="glossaryLink">Lifting
Condensation Level (LCL) represent?
How can it be found on a Skew-T
diagram?
5.
What is CAPE? How can it be found
on a Skew-T diagram?
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=662
Chapter 6: Clouds
ALISON NUGENT AND SHINTARO RUSSELL
Learning Objectives
By the end of this chapter, you should be able to:
1.
Define what a “cloud” is
2.
Describe the importance of cloud
condensation nuclei in cloud formation
3.
Describe the difference between
cumuliform and stratiform cloud classes
4.
Differentiate between and identify
major cloud types including cumulus,
stratus, cirrus, altostratus, etc.
5.
In addition to identifying the cloud,
219
220 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
you should know the general altitude
(low, mid, or high), cloud class
(convective or stratiform) and whether or
not it is a rain cloud
6.
Discuss when/where/why clouds form
Cumulus clouds (Photo by Shintaro Russell).
What is a cloud and how does it form?
A cloud is a collection of suspended particles of water droplets
and/or ice crystals in the atmosphere. The cloud droplets or
crystals are so small that their terminal velocity, the highest
velocity possible as an object falls through a fluid, is negligible.
ATMO 200 • 221
They are falling but they fall so slowly that they appear to be
suspended in the air.
Recall from Chapter 4, the Clausius-Clapeyron diagram. The
line represents the vapor pressure at saturation for a given
temperature. The region to the right of the line represents air that
is sub-saturated, and the region to the left of the line represents
air that is super-saturated. Sub-saturated air has a relative
humidity below 100% and super-saturated air has a relative
humidity above 100%.
222 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
The Clausius-Clapeyron relationship showing that saturated vapor
pressure vs. temperature have an exponential relationship (CC BY-SA
4.0).
Imagine a point on the right side of the line, at T=75 °C, and
e0=12.5 kPa. To become saturated, the air either needs to cool
with the same moisture content (move left on a constant e0 line),
or increase the moisture content (move upward on a constant
T line) until the air meets the saturation curve. To put this
another way, we know that clouds form when the atmosphere is
ATMO 200 • 223
saturated. The list below describes the three ways air parcels can
become saturated.
Clouds can form from
1. Adding Moisture (or mixing with cool moist air)
◦ Sea smoke
◦ Contrails
◦ Cooling towers
◦ Exhaling
2. Cooling by Removing Heat
◦ Radiation fog: clear nights on land
◦ Advection fog: air moving over cold ocean
currents
3. Cooling by Adiabatic Expansion
◦ Upward air motion
◦ Vortices, like wing tip vortices on aircraft,
or tornados
◦ Supersonic flight
Saturation is typically achieved by either adding moisture until
the dew point temperature is equivalent to the temperature or
cooling until the temperature is lowered to the dew point
temperature. In some cases, both moisturizing and cooling can
happen at the same time. The mixture of two unsaturated air
224 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
parcels can even cause saturation in the resulting mixture. For
example, a person’s breath on a cold day or jet contrails can both
form clouds because of this mixing process.
Cloud Condensation Nuclei
When air becomes saturated, excess water vapor in the
atmosphere can condense to form liquid water. However, water
vapor requires a surface on which to condense. The surface
is provided by tiny particulates in the atmosphere known as
aerosols, or more generally, cloud condensation nuclei (CCN).
Aerosols are everywhere in the atmosphere. They can be
composed of earth matter like dust, clay, or soot. They can also
be composed of sea salt from the ocean, black carbon from
fires, or sulfate from volcanic activity. They can be composed
of material from plant or organism life, like sulfates and nitrates
or volatile organic compounds, or even phytoplankton. Anything
that is small enough to be lofted and suspended in the
atmosphere has the potential to be an aerosol. An aerosol that
is hygroscopic, or “water-liking”, has the potential to be a cloud
condensation nuclei.
If there were no aerosols in the atmosphere, relative humidities
well over 100% would be needed for clouds to form. In
laboratory studies with a clean atmosphere (with no aerosols)
relative humidities up to 400% have been measured. In Earth’s
atmosphere, aerosols are abundant, so water vapor does not
struggle to find CCN to condense onto.
Another type of aerosol that helps with the formation of cold
clouds, or clouds composed of ice crystals, are called Ice Nuclei
ATMO 200 • 225
(IN). Dust is a particularly good ice nuclei. Materials with a
hexagonal shape, like ice crystals, also make particularly good
surfaces for ice to form on. A common one used in cloud seeding
is called silver iodide, AgI. We’ll discuss this more in the
following chapter on precipitation.
Cloud Naming Conventions
Once a cloud forms, how do we know what to call it? Clouds
form in many environments and look different depending on
those environments, and depending on whether they’re
composed of liquid droplets or ice crystals. The following
diagram gives a brief overview of the many cloud types, along
with their common abbreviations.
This diagram shows the common cloud types and associated altitudes.
The cloud naming convention involves dividing the atmosphere into
Low, Middle, and High level clouds, and typically cumuliform and
stratiform clouds (CC BY-SA 3.0).
226 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Clouds can be identified based on their appearance, shape, and
altitude. The two primary cloud categories are cumuliform and
stratiform clouds.
By Appearance and Shape
Cumuliform clouds develop as a result of vertical motions by
atmospheric instability. They are convective clouds meaning that
they form in air parcels that are buoyant and are undergoing
convection, which is the transfer of heat or mixing within a
fluid due to warm air rising and cool air sinking. Examples of
cumuliform clouds include cumulus, cumulus congestus, and
cumulonimbus.
Stratiform clouds are horizontally layered clouds. They tend to
spread into wide regions, and take on an appearance of a sheet
or blanket. They typically form when a layer of air is brought to
saturation but is thermodynamically stable, or when a convective
cloud meets a stable layer and spreads out in a layered fashion.
Examples of stratiform clouds include nimbostratus, stratus,
altostratus, cirrostratus, and cirrus.
By Water Phase
Clouds composed of only liquid water droplets are called “warm
clouds”, and typically have clearly defined edges. Low altitude
clouds are usually warm clouds.
Clouds made up of only ice crystals are called “cold clouds” and
typically have fuzzy looking edges. The edges look fuzzy and
not well defined because it takes longer to go from the ice phase
to the vapor phase as compared to warm clouds. This transition
ATMO 200 • 227
time scale results in a larger cloud to dry air transition region.
High altitude clouds are always cold clouds.
Clouds composed of liquid water and ice crystals are called
“mixed phase clouds”. It is difficult to distinguish by eye
whether a cloud is mixed phase. Often the decision comes down
to the height of the cloud, knowing that clouds that extend from
the surface up high in the atmosphere likely have a mixture of
both liquid droplets and ice crystals.
By Altitude
Clouds at a high altitude have the prefix “cirro” or “cirrus”. Due
to the high altitude, the cirrus and cirrostratus are made out of
ice crystals.
Clouds at mid-altitude have the prefix “alto”. They are usually
made out of liquid droplets, but can be a mixture between liquid
droplets and ice crystals.
Low level clouds don’t have a particular prefix.
By Characteristics
The prefix “nimbo” or suffix “nimbus” indicates a precipitating
cloud. Nimbostratus usually have light to moderate
precipitation, whereas cumulonimbus or thunderstorm clouds
have heavy precipitation and sometimes even hail.
Other
The above clouds represent the primary cloud types, but many
other cloud names and cloud types are used in atmospheric
228 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
sciences. A few of the more common cloud names are given
below.
Lenticular clouds are stationary lens-shaped clouds with a
smooth appearance that usually forms over the summit of a
mountain or on the lee wave crest. They are also referred to as
lee-wave clouds or mountain-wave clouds.
Mammatus clouds are formed from downward convection,
typically in the anvil portion of a cumulonimbus. They have
a distinctive look that makes them easy to spot, especially at
sunset. They are made up of hanging pouches that look a little
bit like udders.
Fog
A cloud that forms at or near the ground is called fog. The main
types of fog are upslope, radiation, advection, precipitation or
frontal, and steam fog.
Upslope, radiation, and advection fogs develop due to cooling.
Upslope fog develops as stable, moist air rises over topography
like a hill or mountain. Radiation fog occurs over land as
radiational cooling lowers the air temperature to its dew point
temperature. Radiation fog usually occurs during calm, clear
nights. Advection fog forms as warm, moist air flows over a
colder surface and the air cools to its dew point temperature.
Frontal and steam fogs are formed by the addition of water
vapor. Frontal fog develops when warm raindrops evaporate in a
colder airmass. Steam fog or sea smoke forms as cold air moves
over warmer water.
ATMO 200 • 229
Cloud Identification Examples
The image gallery below contains many different clouds types.
The clouds are labelled and described in their figure captions.
Stratus clouds over Diamond Head. Stratus clouds are a low level cloud
type, composed of liquid droplets. They form in an atmosphere that is
stably stratified, hence the featureless clouds. (Photo by Shintaro
Russell)
230 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Cumulus clouds are a convective low level cloud type. They’re
composed of liquid droplets and form in an unstable, or conditionally
unstable atmosphere. (Photo by Shintaro Russell)
ATMO 200 • 231
Cirrus clouds are a high level cloud type. Cirrus clouds are composed of
tiny ice crystals. (Photo by Shintaro Russell)
232 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Cumulonimbus and Altocumulus clouds. The cumulonimbus cloud is
the cloud that has a large vertical extent in the center of the image. The
altocumulus clouds are on the top and the top right of the image.
Cumulonimbus clouds have the suffix “nimbus” because they’re raining,
and because they extend through so many levels of the atmosphere,
they’re typically mixed phase, having both liquid and ice inside. (Photo
by Shintaro Russell)
ATMO 200 • 233
Cirrocumulus clouds are high level convective clouds. They’re typically
constrained to a thin layer. (Photo by Shintaro Russell)
234 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
As cumulus clouds begin to grow vertically, they’re called cumulus
congestus like these convective clouds in the image. They’re not
cumulonimbus yet, but they’re deeper than the average cumulus. (Photo
by Shintaro Russell)
ATMO 200 • 235
Altocumulus clouds are mid-level cumulus clouds. Altocumulus and
cirrocumulus are often difficult to differentiate from a photograph
because the altitude is hard to gauge. However, these clouds look more
well defined than the cirrocumulus in the prior image, meaning they’re
likely made of liquid droplets rather than ice crystals. (Photo by Shintaro
Russell)
236 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Stratocumulus clouds are low level layered clouds. They’re called strato
– meaning layered, and cumulus – meaning convective because they’re
convective-looking, but confined to a layer. (Photo by Shintaro Russell)
A link to an entire cloud gallery put together by Shintaro Russell
is included here. All of the photos were taken here in Hawaii,
and so many cloud types are represented! Enjoy!
Chapter 6: Questions to Consider
1.
What relationship is shown in
the Clausius-Clapeyron diagram?
2.
Why are CCN important?
3.
What is the difference between
cumuliform and stratiform clouds?
ATMO 200 • 237
4.
Drag and drop the cloud type to its
correct position in the diagram below:
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=250
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=250
Chapter 7: Precipitation
Processes
ALISON NUGENT AND DAVID DECOU
Learning Objectives
By the end of this chapter, you should be able to:
1.
Recall the importance of American
Meteorological Society, cited 2019:
Cloud Condensation Nuclei. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Cloud_condensation_nuclei.]"
class="glossaryLink">cloud
condensation nuclei and aerosols
238
ATMO 200 • 239
2.
Calculate the speed of a falling cloud
droplet and raindrop
3.
Describe the Collision-Coalescence
process
4.
Describe the Ice Phase (WegenerBergeron-Findeisen) process
Raindrops falling on a body of water (CC 0).
Introduction
Sometimes rain feels like a gentle mist but at other times its a
heavy downpour that floods streets and sidewalks. Many times,
clouds cover the skies but never produce any American
Meteorological Society, cited 2020: Precipitation. Glossary of
240 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation at all.
This leads us to question: why does it rain and do raindrop
sizes vary? What is the relationship between raindrops and cloud
droplets, and by what processes do each form? You know that
clouds form by condensation but, apparently, condensation by
itself is a necessary but insufficient condition for rain. We will
explore why this is by examining cloud droplets and raindrops
in more detail.
The average cloud droplet is very small with an average
diameter of about 20 micrometers (μm), which is the same as
20*10-6 m, 0.002 cm, or 0.02 mm. This diameter is about 100
times smaller than your average raindrop.
Pro Tip: 1 micron (μm) is the same as onemillionth of a meter (1*10-6 m). In cloud
microphysics, microns are the standard scale of
measure.
The following image gives a sense of the difference in scale
between raindrops (left), cloud droplets (center), and American
Meteorological Society, cited 2019: Cloud Condensation Nuclei.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Cloud_condensation_nuclei.]"
class="glossaryLink">cloud condensation nuclei (right). The
average raindrop has a diameter of 2 mm, and the average
condensation nucleus has a diameter around 0.0002 mm.
ATMO 200 • 241
Comparison of raindrop, cloud droplet, and condensation nucleus sizes,
given as diameter in mm (Image Created by Britt Seifert).
When considering the volume of the droplets or particles, this
differences quickly grows. The following image shows the
volume of various cloud droplets and rain drops on a log scale.
242 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Type of hydrometeor vs. radius “R”, given in microns, and drop volume
(mm3) on a log scale (CC BY-NC-SA).
Notice how cloud droplet sizes range from 2 μm to 50 μm and
raindrop sizes range from 200 μm to 2500 μm. Liquid drops
exist on a size spectrum from about 1 μm to almost 5,000 μm
(or 0.5 cm). The minimum size for a cloud droplet is effectively
set by the surface tension required to keep the H2O molecules
together. The smaller the droplet, the higher the surface tension
necessary. The maximum size for a raindrop is limited by drop
breakup because when the drop becomes too large, air friction
will break it up into a bunch of smaller droplets.
In general, the only difference between a cloud droplet and a
raindrop is that a raindrop has a non-negligible fall velocity. On
a continuous spectrum of sizes, at some point the gravitational
pull on water drops in the atmosphere becomes large enough not
to ignore. While all drops will fall, the larger the drops are, the
faster they fall.
ATMO 200 • 243
Cloud Droplets
Recall from the previous chapter on clouds that American
Meteorological Society, cited 2019: Cloud Condensation
Nuclei. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Cloud_condensation_nuclei.]" class="glossaryLink">cloud
condensation nuclei (CCNs) are required for water vapor to
condense onto. Many CCNs are hygroscopic, meaning they tend
to absorb moisture, so condensation may start before the relative
humidity reaches 100%. For example, when condensation
occurs on salt particles, which are extremely hygroscopic,
condensation can begin at 80% relative humidity or lower.
Imagine there are many CCNs of differing sizes in a body of
humid, but unsaturated air. If the air were to be lifted by a
mountain or in a rising thermal, it would cool, and the relative
humidity would increase. As the air nears American
Meteorological Society, cited 2019: Saturation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Saturation.]"
class="glossaryLink">saturation,
condensation will begin to occur on the largest and most
hygroscopic CCN. At some later time, a cloud of many small
cloud droplets, far too small to fall at any significant speed, will
form.
The American Meteorological Society, cited 2020: Terminal Fall
Velocity. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Terminal_fall_velocity.]"
class="glossaryLink">terminal velocity of a falling cloud
244 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
droplet (with radius “r” less than 40 μm) is given by the
following equation from Stoke’s Drag Law:
where “r” needs to be expressed in meters. A simple calculation
will show that it takes hours, if not days, for a cloud droplet to
fall from even low altitudes. The friction provided by the air or
even tiny upward air currents will keep cloud droplets suspended
for long periods.
American Meteorological Society, cited 2020: Terminal Fall
Velocity. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Terminal_fall_velocity.]"
class="glossaryLink">Terminal velocity implies that a steady
state has been reached in the fall velocity with a balance between
the downward gravitational force on the drops and the upward
frictional drag of air.
Raindrops
For raindrops, a different equation is used to approximate the fall
speed. For spherical raindrops,
where ρ0 is a reference value of density, typically 1.2 kg m-3
and ρair is the density of air where the raindrop exists. Again,
“r” is the radius of the drop, given in meters. High up in the
atmosphere when ρair is small, the speed of a falling raindrop,
ATMO 200 • 245
wT, rain, will be faster than near the surface when ρair is similar
magnitude to ρ0. As the air density increases, the frictional drag
on a drop also increases.
Note that this equation for raindrops is a vast simplification
because raindrops are not typically spherical shaped. As they
fall, the passing air deforms them into pancake shaped drops.
However, this equation provides an approximation for fall
velocity.
So how can cloud droplets grow to form raindrops? The
condensation process is not enough and is far too slow to
produce raindrops. From a volumetric view, it takes 1 million
cloud droplets (10 μm radius) to combine together to make
one single raindrop (1000 μm = 1 mm radius). There has to
be another faster process by which cloud droplets can grow or
combine together to become large and heavy enough to fall.
We will discuss two primary rain formation theories.
1. Collision-Coalescence Process
2. Ice Phase Processes (Wegener-Bergeron-Findeisen)
Collision-Coalescence Process
In warm clouds, where all of the cloud droplets are liquid,
the collision-coalescence process is the primary mechanism
responsible for producing American Meteorological Society,
cited 2020: Precipitation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
246 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">precipitation. This is thought to be the
case especially over tropical oceans. The collision-coalescence
process is exactly as it sounds: cloud droplets collide and
coalesce or stick together. Larger cloud droplets have slightly
higher terminal velocities, because they have a smaller surfacearea-to-weight ratios. This advantage allows them to fall faster
and collide with smaller cloud droplets. Sometimes the cloud
droplets will stick together and coalesce to form a larger droplet.
This begins a positive feedback where these larger droplets then
fall even faster, collide with even more smaller droplets in their
path, and aggregate more and more cloud droplets together.
However, note that collision between cloud droplets does not
always mean that coalescence will occur. Sometimes droplets
will bounce apart during collision if their surface tensions are
too strong. For collision-coalescence to begin, a cloud needs to
have a wide distribution of cloud droplet sizes. This can occur
from a variation in CCN type—for example, sea salt aerosols are
particularly large—or from random collisions between droplets.
The total amount of liquid water in a cloud as well as the time
that a cloud droplet spends inside of a cloud influences how
large it can grow through the collision-coalescence process. The
cloud height is of course a factor here, but its a little more
complicated than that. Rising motion in a cloud will slow the
downward speed of a falling droplet. This can act to increase the
amount of a time a cloud droplet spends in a cloud, as well as the
size it will grow. Let’s cover a few examples.
Deep cumulus clouds with convective updrafts tend to produce
larger raindrops because upward motion is strong and droplets
ATMO 200 • 247
have a long time in the cloud to grow. In fact, the droplets need
to become sufficiently large in order for their fall velocity to
overcome the updraft velocity.
On the other hand, stratus clouds are typically not very thick
and have weak updrafts, so droplets in these clouds don’t spend
a long time in the cloud itself, and therefore are not be able to
grow very large. If there is moist air below the stratus cloud,
the drops may reach the ground as a light drizzle. However, if
there is dry air below the stratus cloud, the drops may evaporate
before they are able to reach the ground.
To summarize, in warm clouds, cloud droplets grow to American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation sized
drops through the collision-coalescence process. The most
important factor in raindrop production is the liquid water
content of a cloud. Assuming the cloud has sufficient water,
other factors that affect raindrop production are: thickness of
the cloud; strength of updrafts within the cloud; cloud drop
distribution of sizes; and difference in electric charge of the
droplets and the cloud itself. Thin stratus clouds with weak
vertical motion may produce weak drizzle, if any, while tall
cumulus clouds with strong updrafts can produce heavy rain
showers. The following image illustrates the collisioncoalescence process of raindrop production.
248 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
The collision-coalescence process occurring in warm clouds. The left
image shows the importance of a range of cloud droplet sizes to initiate
the collision-coalescence process while the image at right shows the
acceleration of the process once a raindrop forms (Image Created by
Britt Seifert).
Ice Phase Process
Ice crystals forming on a window at 30,000 feet (CC BY-SA 3.0).
Outside of the tropics, the ice phase process of rain formation is
ATMO 200 • 249
the primary mechanism producing most of the worlds American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation. The
ice phase process occurs in cold clouds or clouds with
temperatures below 0°C. To understand why, we need to know
something about freezing of liquid water droplets.
Supercooled Water and Ice Nuclei
Supercooled water is liquid water that exists below the freezing
point of water (below 0°C). Similar to how cloud droplets need
a surface on which to condense, ice crystals also need a nucleus
or ice embryo to freeze. Without an ice nucleus, liquid water
drops can remain liquid in temperatures as low as -40°C. Once
beyond -40°C, all hydrometeors (water particles) will exist in
the solid state. Typically the distribution of liquid and solid
hydrometeors in a cloud looks like the following image.
250 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
A simple diagram showing the distribution of liquid and solid phase
hydrometeors in a mixed phase cloud (CC BY-SA 3.0).
At low elevations above freezing (region 4), the hydrometeors
in the cloud exist as liquid droplets. Above the freezing level
(region 3), supercooled liquid droplets exist. Above that, some
liquid droplets begin to freeze, and both liquid and ice phase
hydrometeors co-exist (region 2). Finally, above some level
where the temperature is cold enough, all hydrometeors will
exist in their solid state (region 1).
When liquid water droplets freeze without any sort of nucleus,
this is known as homogenous or spontaneous freezing. While
this occurs within a large body of freshwater at temperatures
ATMO 200 • 251
slightly below 0°C, cloud droplets will not freeze spontaneously
until temperatures are -40°C or lower.
For droplets to freeze spontaneously, enough molecules within
the droplet must form a rigid pattern and become a tiny ice
structure known as an ice embryo. When this embryo grows
sufficiently large, at a certain size it will act as an ice nucleus,
which are described below. The other molecules in the droplet
then become attached to the ice structure and the entire droplet
freezes.
Tiny ice embryos are able to form when water drops just below
freezing, but typically at these temperatures there is enough
thermal agitation to weaken their structure and break them apart.
At lower temperatures, there is less thermal motion, and ice
embryos have a better chance of growing large enough to freeze
the surrounding water. When you have larger volumes of water,
ice embryos have a better chance of growing large enough to
freeze the surrounding liquid before being broken up, but this
becomes more and more difficult with smaller volumes of water.
Only the largest cloud droplets can freeze spontaneously without
a nucleus at temperatures below -40°C. In most cases, American
Meteorological Society, cited 2020: Ice nucleus. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Ice_nucleus.]" class="glossaryLink">ice nuclei are
required for ice crystals to form in sub-freezing clouds.
Just as CCNs are required for liquid cloud droplets to form,
ice crystals form on particles called American Meteorological
Society, cited 2020: Ice nucleus. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
252 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Ice_nucleus.]" class="glossaryLink">ice nuclei (IN).
Particles serve as effective IN if they have similar geometry to
an ice crystal, for example, ice itself is an effective IN. There
are not many IN in the atmosphere, especially at temperatures
above -10°C, but certain types of particles become active IN
with lower temperatures. For example, dust can be an effective
IN. American Meteorological Society, cited 2020: Ice nucleus.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Ice_nucleus.]"
class="glossaryLink">Ice nuclei are rare compared to
hygroscopic American Meteorological Society, cited 2019:
Cloud Condensation Nuclei. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Cloud_condensation_nuclei.]"
class="glossaryLink">cloud
condensation nuclei.
Some IN allow water vapor to immediately become solid ice
when they come in contact together. These are known as
deposition nuclei because the water vapor changes phase
directly into solid ice without becoming liquid first (phase
change from gas to solid is called deposition). IN that are
effective at causing the freezing of supercooled liquid droplets
are called freezing nuclei. Some freezing nuclei must be
immersed in a liquid drop in order to freeze it, while others
are effective at inducing condensation and then freezing. Many
freezing nuclei will cause supercooled droplets to freeze as soon
as they collide, which is called contact freezing, and these
nuclei are referred to as contact nuclei. These different freezing
methods are outlined in the figure below.
ATMO 200 • 253
Four primary mechanisms that form ice in the atmosphere:
homogeneous freezing; deposition nucleation; immersion freezing; and
contact freezing (CC BY-SA 4.0).
To summarize, cloud droplets may freeze spontaneously, but
only at very low temperatures. American Meteorological
Society, cited 2020: Ice nucleus. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Ice_nucleus.]" class="glossaryLink">Ice nuclei can help the
growth of ice crystals, but they are not naturally abundant.
Saturation Vapor Pressure
So, we have cold clouds that contain many more liquid cloud
droplets than ice crystals, even at sub-freezing temperatures, and
these particles are not large/heavy enough to precipitate out of
the cloud. How do we get rain and snow out of the ice-crystal
process then?
254 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Imagine a cloud with super cooled liquid water and saturated
air. At American Meteorological Society, cited 2019: Saturation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation, the liquid droplets are in
equilibrium with the water vapor in the air. The number of water
molecules leaving and entering the surface of the liquid droplets
are equivalent. Now imagine that an ice crystal forms by one of
the processes described above. In the below-freezing portion of a
cloud, this ice crystal is surrounded by many liquid supercooled
droplets. Because the American Meteorological Society, cited
2019: Saturation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation vapor pressures with respect to
liquid and ice are slightly different, the presence of this new ice
crystal has a big impact on the cloud.
ATMO 200 • 255
The saturation vapor pressure with respect to liquid (blue) and with
respect to ice (purple). The difference is shown at the top (CC
BY-NC-SA).
With respect to liquid, the liquid droplets were at American
Meteorological Society, cited 2019: Saturation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Saturation.]" class="glossaryLink">saturation. But now,
with respect to ice, the ice crystal is in an environment that
is supersaturated. You can think of this as the following: it is
easier for water molecules to escape a liquid surface through
evaporation than to escape a solid surface. This means that there
256 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
will be many more molecules escaping the liquid water surface
at a given temperature and will require more water vapor around
it in order to keep the droplet in American Meteorological
Society, cited 2019: Saturation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Saturation.]" class="glossaryLink">saturation. At the same
temperature, the American Meteorological Society, cited 2019:
Saturation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation American Meteorological
Society, cited 2019: Vapor pressure. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Vapor_pressure.]" class="glossaryLink">vapor pressure above a
surface of water is greater than the American Meteorological
Society, cited 2019: Saturation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Saturation.]"
class="glossaryLink">saturation
American
Meteorological Society, cited 2019: Vapor pressure. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Vapor_pressure.]" class="glossaryLink">vapor pressure
above a surface of ice.
This difference in American Meteorological Society, cited 2019:
Saturation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]"
class="glossaryLink">saturation American Meteorological
Society, cited 2019: Vapor pressure. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Vapor_pressure.]" class="glossaryLink">vapor pressure with
respect to water and ice causes water vapor molecules to deposit
ATMO 200 • 257
from the environment onto the ice crystal. Because the vapor
molecules are being removed from the environment around the
liquid droplet, the American Meteorological Society, cited 2019:
Vapor pressure. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Vapor_pressure.]"
class="glossaryLink">vapor pressure with respect to the water
surface decreases. This throws the water droplets out of
equilibrium with their surroundings, causing them to evaporate,
replenishing the removed water vapor from the environment.
This provides additional moisture for the ice crystal, allowing it
to grow at the expense of the liquid droplets.
This process is called the Wegener-Bergeron-Findeisen (or
more generally, the ice phase) process. Ice crystals in a subfreezing region of a cloud will grow larger at the expense of
surrounding water droplets.
Falling Ice Crystals
Imagine this happening throughout a large cloud. The water
vapor within the cloud as well as the water vapor from
evaporating supercooled liquid droplets provides a continuous
source of moisture for ice crystals, allowing them to grow
rapidly. Eventually, these ice crystals become large enough to
fall. The same issues exist with updrafts and rising air, but at
some point the crystals will fall faster than the updrafts within
the cloud.
Sometimes, ice crystals collide with nearby supercooled droplets
in the cloud, causing them to freeze onto the crystal as ice. The
258 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
ice crystal will grow larger and larger as it collides into more
droplets, this is called riming or American Meteorological
Society, cited 2020:Accretion. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Accretion.]" class="glossaryLink">accretion. This forms an
icy clump called American Meteorological Society, cited
2020: Graupel. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Graupel.]"
class="glossaryLink">graupel (also known as snow pellets),
which may break apart into tiny ice particles as it collides with
more droplets. These splinters may form American
Meteorological Society, cited 2020: Graupel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Graupel.]" class="glossaryLink">graupel of their own as
they collide into other droplets, which in turn may also splinter,
causing a chain reaction.
In clouds that are colder, ice crystals may collide together and
break apart into smaller ice particles, which act as tiny seeds
that can freeze supercooled droplets on contact. This could also
cause a chain reaction that produces many ice crystals. As these
ice crystals fall, they can collide and stick together. This process
of collision and sticking is called American Meteorological
Society, cited 2020: Aggregation. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Aggregation.]" class="glossaryLink">aggregation. A fully
grown ice crystal is what we call a American Meteorological
Society, cited 2020: Snowflake. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Snowflakes.]" class="glossaryLink">snowflake.
ATMO 200 • 259
The growth of ice crystals occurs during its descent through aggregation
(CC 0).
There must be many times more water droplets in a cloud than
ice crystals for ice crystals to get large enough to fall as
snow—on the order of 100,000:1 to 1,000,000:1.
Precipitation Types
The ice phase process accounts for most global American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation. The
term American Meteorological Society, cited 2020:
Precipitation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation refers to all types of
American Meteorological Society, cited 2020: Precipitation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation—from fog, rain, snow, hail,
etc. Anything composed of water that is falling in the
atmosphere can be called American Meteorological Society,
260 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
cited 2020: Precipitation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation. However, we know that not
everywhere in the world gets snow all the time. While the ice
phase process helps with American Meteorological Society,
cited 2020: Precipitation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation formation, many things can
happen to falling drops along their journey from the cloud to the
ground. Here are a few examples.
Rain: Ice crystals melt before they hit the ground.
Snow: Ice crystals collide and stick, forming a fully grown ice
crystal, and falling to the ground as a American Meteorological
Society, cited 2020: Snowflake. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Snowflakes.]" class="glossaryLink">snowflake.
American Meteorological Society, cited 2020: Graupel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Graupel.]"
class="glossaryLink">Graupel: Ice crystals collide and stick
with other ice crystals forming clumps of snow called American
Meteorological Society, cited 2020: Graupel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Graupel.]" class="glossaryLink">graupel.
Sleet: A mixture of rain and snow, formed by partial melting.
ATMO 200 • 261
Freezing Rain: Supercooled liquid rain that freezes on impact
with the surface.
Ice Pellets: Ice crystals melt before they hit the ground, refreeze
in a cold layer, usually just above the surface, and end up falling
as frozen rain drops.
Hail: Ice crystals that repeatedly pass through a supercooled
liquid region of cloud where riming builds up on the
hydrometeor. The formation of hail requires strong updrafts and
a relatively long time inside of a cloud.
As you can see, the journey of an ice crystal after formation
is not always straightforward and depends strongly on the
environmental conditions, especially temperature. In the
following chapters (especially chapter 12), we will learn how
airmasses and fronts combine together regularly in Earth’s
atmosphere to create conditions that are conducive to all types
of American Meteorological Society, cited 2020: Precipitation.
Glossary
of
Meteorology.
[Available
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation.
online
at
262 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Precipitation changes associated with the passage of a warm front
(Public Domain).
The above image provides an example of how a American
Meteorological Society, cited 2020: Warm Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Warm_front.]" class="glossaryLink">warm front creates
temperature gradients in the atmosphere that produces American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation types
from rain, freezing rain, sleet, and snow depending on your
location with respect to the American Meteorological Society,
cited 2020: Front. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front.
ATMO 200 • 263
Chapter 7: Questions to Consider
1.
Which rain formation
process produces most of the worlds
American Meteorological Society, cited
2020: Precipitation. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Precipitation.]"
class="glossaryLink">precipitation?
What type of cloud does this process
occur in?
2.
What factors determine how large a
cloud droplet can grow through collisioncoalescence?
3.
Match the ice nucleation methods:
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=304
4.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=304
264 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=304
Chapter 9: Weather Reports and
Map Analysis
ALISON NUGENT AND DAVID DECOU
Learning Objectives
By the end of this chapter, you should be able to:
1.
Describe a few global atmospheric
observation networks
2.
Define the meaning of pressure and
temperature lines on a map
3.
Discuss the importance of American
Meteorological Society, cited 2020:
Synoptic. Glossary of Meteorology.
[Available online at
265
266 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://glossary.ametsoc.org/wiki/
Synoptic.]"
class="glossaryLink">synoptic maps,
and the information required to make one
High resolution surface map (Public Domain).
Introduction
American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">Weather encompasses the state of the
atmosphere at any given time, and it is in a constant state of
ATMO 200 • 267
change. Temperature, pressure, humidity, wind, American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation, cloud
cover, visibility—this type of data is constantly being collected
around the globe, both at the surface and aloft in the upper
atmosphere. How is this data collected and where does it come
from? How is it shown on a map? What is the purpose of
plotting American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather conditions on a map if they are
changing all the time? This chapter serves as a brief introduction
to surface American Meteorological Society, cited 2020:
Synoptic. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Synoptic.]"
class="glossaryLink">synoptic
American
Meteorological
Society, cited 2019: Weather. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather maps and interpreting the data
plotted on them. Simplified American Meteorological Society,
cited 2019: Weather. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather charts are used frequently on TV,
showing locations of high and low pressure systems, fronts, and
storm systems. You’re likely familiar with this material already,
just from your day to day experiences.
268 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorological Reports and Observations
The World Meteorological Organization (WMO) is a branch
of the United Nations and it sets global American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather observation
standards to be used by countries across the globe. American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">Weather systems span
countries and continents so American Meteorological Society,
cited 2019: Weather. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather observations have to be
synchronized to get an accurate big-picture view (a American
Meteorological Society, cited 2020: Synoptic. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Synoptic.]"
class="glossaryLink">synoptic view) of the American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather at a given time.
Upper air and surface observations are taken at specific times
in Coordinated Universal Time (UTC) so that they can be
coordinated simultaneously across different time zones. For
example, most upper-air observations are taken at 00 and 12
UTC, but surface observations are typically taken more
frequently. American Meteorological Society, cited 2020:
Synoptic. Glossary of Meteorology. [Available online at
ATMO 200 • 269
http://glossary.ametsoc.org/wiki/Synoptic.]"
class="glossaryLink">Synoptic American Meteorological
Society, cited 2019: Weather. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather maps put together these
observations.
Observations are reported using internationally standardized
codes. One of the most commonly used codes is METAR, which
comes from the French MÉTéorologique Aviation Régulière, or
Meteorological Terminal Aviation Routine. It is a summary of
surface American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather conditions reported at hourly or
half hourly intervals depending on the station. METAR is
provided by airport terminals for the purposes of aviation
meteorology. In general, METAR includes reports of wind speed
and direction, visibility, cloud layers at different levels of the
atmosphere, surface temperature and dew point temperature,
air pressure, as well as American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather conditions such as American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation or
thunderstorms near the station. SPECI is a special non-routine
form of METAR, provided when airport American
Meteorological Society, cited 2019: Weather. Glossary of
270 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather conditions
change significantly. You are not required to translate or
memorize METAR for the purposes of this class, but it is useful
to recognize it because it is very commonly used. Translations
for parts of METAR code can be easily found online.
METAR: PHNL 072153Z 05012KT 10SM FEW025 FEW050
BKN200 31/17 A3006 RMK AO2 SLP180 T03060167
Above is an example of METAR, taken from Honolulu
International Airport. “PHNL” is the ICAO airport code for
Honolulu International. The “072153Z” indicates that the report
was given on the 7th of the month (August 7th) at 2153Z
(Universal Time), which is 11:53 AM local time. The
“05012KT” is the wind report, and indicates that winds are
blowing at 12 knots from 50°, which is roughly from the
northeast. The “10SM” indicates that visibility is 10 statute
miles, which is another way of saying that visibility on the
runway is clear. If visibility is 10 statute miles or greater, it
is always reported as just “10SM”. The “FEW025 FEW050
BKN200” are reports of different cloud heights, which are
important for aviation purposes. This report says that there are
is a layer of a “few” clouds at 2,500 and 5,000 feet, with a
layer of “broken” clouds at 20,000 feet. The “31/17” is telling
us that the temperature is 31°C with a dew point temperature of
17°C. The “A3006” gives the station air pressure at 30.06 in Hg
(inches of Mercury, this unit is still primarily used in aviation).
The “RMK” denotes the remarks section of a METAR report
where additional remarks and information about the American
ATMO 200 • 271
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather are provided.
The “AO2” is a code that indicates that the airport observing site
is automated and contains a American Meteorological Society,
cited 2020: Precipitation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation sensor. Some sites are not
automated or do not have a American Meteorological Society,
cited 2020: Precipitation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation sensor so there are different
codes to denote this. Automated reports can also have nonautomated remarks added to them. The “SLP180” gives the sea
level pressure as 1018.0 mb. And the “T03060167” gives a
more accurate temperature and dew point temperature reading. It
reports that the temperature is actually 30.6°C and the dew point
temperature is actually 16.7°C.
Weather Observation Locations
Surface American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather observations include automated
observations from Automated Surface Observing System
(ASOS) sites in the United States, as well as hourly observations
from airports around the world, reported as METAR. Manual
and ship observations are also made at specific times. Surface
272 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
observations of the American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather are important for providing
information about the American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather conditions that we experience
here on the ground. These conditions include temperature, dew
point temperature, wind speed and direction, air pressure, cloud
cover, visibility, and American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather conditions such as American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation or
thunderstorms.
Although surface American Meteorological Society, cited 2019:
Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather observations are important, they
only tell a part of the full story. Just as ripples on the surface
of a river can be a sign of what is happening below, surface
observations can give an idea of what is happening above. Much
of the American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather that happens at or near the
ATMO 200 • 273
ground is strongly affected by conditions higher up in the
atmosphere.
Observations of the upper atmosphere are taken by American
Meteorological Society, cited 2020: Radiosonde. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiosonde.]"
class="glossaryLink">radiosonde packages, which are
carried aloft by American Meteorological Society, cited 2019:
Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather balloons. These American
Meteorological Society, cited 2020: Radiosonde. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiosonde.]"
class="glossaryLink">radiosonde
observations (RAOBs) provide soundings of the upper-air
environment, giving information about temperature, humidity,
and pressure at vertical levels throughout the atmospheric
column. Recall from Chapter 5 that temperature and humidity
aloft are usually plotted against pressure in a thermodynamic
diagram called a Skew-T. Some radiosondes can infer wind
data at different heights—these are called rawinsondes. When
these instrument packages are dropped from an aircraft, they are
called dropsondes. All of this data is accumulated, organized,
tested, and stored in computers at governmental centers such
as the European Centre for Medium-Range American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">Weather Forecasts (ECMWF). Other
274 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
data collecting systems include American Meteorological
Society, cited 2019: Weather. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather radar (such as the NEXRAD
network in the United States) and satellite, as well as vertical
wind profilers and Radio Acoustic Sounding Systems (RASS).
The following figures show locations of data collected by
ECMWF over a 6-hour time period. The first figure shows
surface observation locations located on land, the second shows
surface observations over the ocean, and the third shows the
upper air sounding observational network.
Surface observation locations for temperature, humidity, winds, clouds,
precipitation, pressure, and visibility (CC BY-NC-SA 4.0).
ATMO 200 • 275
Buoy observation platforms for surface temperature and winds over the
ocean (CC BY-NC-SA 4.0).
Upper air observation and sounding locations for temperature, pressure,
and humidity aloft. Stull Figure 9.4 (CC BY-NC-SA 4.0).
Sea-Level Pressure Adjustment
American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
276 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">Weather stations exist at many altitudes.
Because air pressure decreases with height, American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather stations in cities
at high elevations will report far lower air pressures than cities at
lower elevations. Air pressure varies vertically far more than it
does horizontally. To make air pressure comparable, the station
pressure needs to be adjusted to the pressure it would have if
the station were at sea level. If this were not done, pressure
differences between nearby surface stations would be dominated
by their difference in elevation, and a surface pressure map
would more closely resemble a map of elevation rather than
a map of atmospheric pressure. By correcting for altitude
differences, American Meteorological Society, cited 2020:
Synoptic. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Synoptic.]"
class="glossaryLink">synoptic maps show mean sea level
pressure, which is used to show low and high pressure centers
near the surface.
Synoptic Weather Maps
Surface American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather observations taken at many
American Meteorological Society, cited 2019: Weather.
ATMO 200 • 277
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather stations at a given time can be
shown on a American Meteorological Society, cited 2019:
Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather map through the use of a station
plot model, an example of which is given below. Station plot
models typically provide wind information through wind barbs,
as well as current American Meteorological Society, cited 2019:
Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather conditions, sea level pressure,
temperature, dew point temperature, visibility, and cloud cover.
Sometimes additional information such as pressure tendency,
cloud type, and American Meteorological Society, cited 2020:
Precipitation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation amounts are also given.
When reading wind barbs, keep in mind that the flag always
points in the direction that the wind is coming from. You can
think of it as an arrow flying from a bow, the flag represents
the feathers, which are at the back of the arrow in the direction
the arrow is coming from. The tip is the direction the arrow is
flying toward. More information on wind barb and cloud cover
symbols are provided below.
Sea level pressure is typically given in three digits, with the
last digit being the nearest tenth. The initial 9 or 10 is left out.
278 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
So, “147” is actually 1014.7 mb and “998” would be 999.8 mb.
When reading station plot pressure, mentally place a decimal
point to the left of the third digit, and use your intuition and the
pressure of nearby stations in order to decide if the pressure has
a leading 9 or 10.
Example station plot model (Public Domain).
The table below provides information for wind barbs. Two
concentric circles with no lines indicates calm wind and a line
with no barbs is 1-2 speed units. A half line is 5, a full line is 10,
and a triangle is 50. On many American Meteorological Society,
cited 2019: Weather. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather maps, the unit for winds is given
in knots, although different American Meteorological Society,
cited 2019: Weather. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather maps can use different units so it
ATMO 200 • 279
is always good to check. The lines and triangles are read a bit
like roman numerals—just add them up!
Wind barb chart (CC BY-NC-SA 4.0).
The table below provides sky cover information, which is
typically shown in the circular center of the station plot model.
Depending on the symbol type, it can indicate clear sky or
various levels of overcast skies.
280 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Sky cover chart. Stull Table 9-10 (CC BY-NC-SA 4.0).
Map Analysis
Surface observations as described above are plotted on a map in
the form of station plot models, as in the figure below. When
looking at the map of raw data below, you can get an idea
ATMO 200 • 281
of the prevailing wind direction in different parts of the US,
which areas are experiencing widespread rain, which areas are
overcast, and from the wind direction patterns you can even
infer centers of high and low pressure. However, much of this
information is not very clear without doing some analysis on the
map. Map analysis can either be done by hand or by computer,
and involves the drawing of contours, or American
Meteorological Society, cited 2020: Isopleths. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Isopleths.]"
class="glossaryLink">isopleths (lines of equal value) to
connect areas of constant air pressure and temperature at the
surface. Aloft, contours may be drawn to show areas of constant
height, constant humidity, constant wind speed, and other
parameters of interest at constant pressure levels. Map analysis
also includes drawing and labeling boundaries in the
atmosphere, such as fronts or dry lines to show the locations
and movement of air masses. Fronts will be covered in a later
chapter. Here, we will focus on lines of constant pressure,
American Meteorological Society, cited 2020: Isobars.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars, and lines of constant
temperature, American Meteorological Society, cited 2020:
Isotherms. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Isotherms.]"
class="glossaryLink">isotherms.
282 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Map with station plot models (CC BY-NC-SA 4.0).
Isopleths
The below figure shows a grid of equally spaced temperature
values in °C. When analyzing the temperature on a map, you
draw American Meteorological Society, cited 2020: Isopleths.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Isopleths.]"
class="glossaryLink">isopleths of constant temperature:
American Meteorological Society, cited 2020: Isotherms.
Glossary
of
Meteorology.
[Available
online
at
ATMO 200 • 283
http://glossary.ametsoc.org/wiki/Isotherms.]"
class="glossaryLink">isotherms. The main purpose of this is to
separate areas of warmer air from cooler air, and to get an idea
of how quickly the temperature changes across a horizontal
distance, which is also known as the temperature gradient.
American Meteorological Society, cited 2020: Isotherms.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Isotherms.]"
class="glossaryLink">Isotherms are typically drawn every 5°C
on the 5’s – for example, 5°C, 10°C, 15°C, 20°C, 25°C, and so
on. Standard conventions are used for contouring on different
map surfaces, for example, American Meteorological Society,
cited 2020: Isotherms. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Isotherms.]"
class="glossaryLink">isotherms are usually dashed and shown
in red. You will not need to know these conventions for this
class, but it may still come in handy to be aware.
In looking at the below temperature field, you can see that there
are many values that do not line up exactly with the lines you
need to draw. Many values lie in between contours, such as 4°C
and 16°C. You will need to interpolate data, meaning you will
have to infer where a datapoint is based on the data around it.
Starting from the upper left corner, imagine that you are going
to draw your 5°C isotherm. Five lies much closer to 4 than 16,
so you will start your isotherm close to the 4. 5°C will also
lie in between 3 and 15 °C, but slightly farther away from the
3 value, so you will slope your isotherm down slightly. Five
degrees centigrade will also lie in between 2 and 7 °C, so you
will continue your isotherm to the right. However, it will slope
284 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
down even further, because 5 lies closer to 7 than to 2. Your
isotherm will then cross directly through the 5°C data point and
slope back upward slightly between the 3 and 6 and directly
through the middle between the 4 and 6 and back up through the
5 in the upper right hand corner.
Blank temperature field (CC BY-NC-SA 4.0).
The complete contoured example of this temperature field is
given below. The atmosphere is a continuous fluid, so any fields
that you are contouring (pressure, temperature) will never have
values that jump or suddenly end. Contours will either be closed
(both ends will connect) or they will extend to the edge of the
map. They will never cross or end suddenly. If two contours
were to cross, it would mean that one place has two different
ATMO 200 • 285
air temperatures at one time, which is impossible. Areas where
American Meteorological Society, cited 2020: Isotherms.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Isotherms.]"
class="glossaryLink">isotherms are close together indicate a
strong temperature gradient, which may be indicative of a frontal
zone.
Completed temperature field with isotherms (CC BY-NC-SA 4.0).
Isobars
Pressure fields are analyzed by drawing American
Meteorological Society, cited 2020: Isobars. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars, or lines of constant air
286 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
pressure. The surface mean sea level pressure is analyzed in
much the same way as the American Meteorological Society,
cited 2020: Isotherms. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Isotherms.]"
class="glossaryLink">isotherms
above.
However,
the
conventions are different. Typically, pressure is analyzed every 4
mb and centered on 1000 mb. American Meteorological Society,
cited 2020: Isobars. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">Isobars are typically drawn as solid black
lines. The mean sea level pressure field is analyzed in order
to identify areas of high and low pressure, as shown in the
figure below. Areas where American Meteorological Society,
cited 2020: Isobars. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars are drawn close together indicate
places where the American Meteorological Society, cited 2020:
Pressure Gradient Force. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Pressuregradient_force.]" class="glossaryLink">pressure gradient force
is strong. A stronger American Meteorological Society, cited
2020: Pressure Gradient Force. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Pressuregradient_force.]" class="glossaryLink">pressure gradient force
is indicative of stronger wind speeds. As you will learn later,
winds tend to flow counterclockwise around areas of lower
pressure, and clockwise around areas of high pressure in the
Northern Hemisphere.
ATMO 200 • 287
Surface map with isobars and station plot models (Public Domain).
Because American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather information is shared across
continents and across country borders, it is important to maintain
standards such that everyone effectively speaks the same
language. American Meteorological Society, cited 2019:
Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">Weather is one of the greatest
international collaborations ever. If it weren’t for China and
Russia sharing their American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather information with the US, our
288 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
long range forecasts would be more inaccurate because we
wouldn’t know the initial conditions of the atmosphere upstream
of the North American continent.
Chapter 10: Atmospheric Forces
and Wind
ALISON NUGENT AND SHINTARO RUSSELL
Learning Objectives
By the end of this chapter, you should be able to:
1.
Define wind and why it occurs
2.
Use u, v, and w to describe motion
3.
Describe the five physical forces that
can act on a parcel of air
4.
Draw force diagrams for geostrophic
wind, gradient wind, and wind in the
American Meteorological Society, cited
289
290 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
2019: Atmospheric boundary layer.
Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/
wiki/Atmospheric_boundary_layer.]"
class="glossaryLink">atmospheric
boundary layer (ABL)
5.
Compute the speed of a geostrophic
wind, and be able to qualitatively
estimate the wind speed from American
Meteorological Society, cited 2020:
Isobars. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Isobars.]" class="glossaryLink">isobars
ATMO 200 • 291
Map of wind barbs and isobars (Public Domain).
Introduction
Wind is the movement of air relative to the Earth’s surface.
As with all moving things, it is caused by a force acting on it.
Force is a pull or push that changes the resting state, motion,
or direction of an object. Force can also cause objects to
accelerate. Human skin can sense wind when an uncountable
number of molecules collide with us as they flow along in the
air, and sense the pressure changes in the air flow.
Main forces
There are five forces that influence the speed or direction of
horizontal winds.
292 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
1. American Meteorological Society, cited 2020:
Pressure Gradient Force. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Pressure-gradient_force.]"
class="glossaryLink">Pressure gradient force
2. American Meteorological Society, cited 2020:
Advection. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/
Advection.]" class="glossaryLink">Advection
3. American Meteorological Society, cited 2020:
Centrifugal Force. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Centrifugal_force.]"
class="glossaryLink">Centrifugal force
4. American Meteorological Society, cited 2020:
Coriolis Force. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/
Coriolis_force.]" class="glossaryLink">Coriolis force
5. American Meteorological Society, cited 2020:
Turbulent Drag. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/
Drag_law.]" class="glossaryLink">Turbulent drag
Remember from Chapter 1 that according to the Cartesian
coordinates, x points east, y points north, and z points upwards.
To define wind, we use wind components u, v, and w which
correspond to the x, y, and z directions. These wind components
are used in equations of motion used to predict wind to designate
direction in a three-dimensional plane. We’ll go through each of
ATMO 200 • 293
the five forces one by one to discuss how they affect wind speed
and/or direction.
Pressure Gradient Force
A pressure gradient (PG) is a change in pressure over a
distance. Therefore, the units of the American Meteorological
Society, cited 2020: Pressure Gradient Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Pressure-gradient_force.]" class="glossaryLink">pressure
gradient force are Pascals per meter (P m-1). The pressure
gradient can be calculated simply as the change in pressure
divided by the distance over which that change occurs. The
size or strength of the pressure gradient determines the size or
strength of the force that results from it.
The American Meteorological Society, cited 2020: Pressure
Gradient Force. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Pressuregradient_force.]" class="glossaryLink">pressure gradient
force (PGF) is a force from high to low pressure over a distance.
Without differences in pressure, there would be no wind because
there would be nothing to accelerate airflow. In Chapter 9 we
learned about American Meteorological Society, cited 2020:
Isobars. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars, or lines of constant pressure.
294 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
When American Meteorological Society, cited 2020: Isobars.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars are tightly packed, we know that
there is a strong pressure gradient or large change in pressure
over a relatively short distance. A strong pressure gradient
results in a large American Meteorological Society, cited 2020:
Pressure Gradient Force. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Pressuregradient_force.]" class="glossaryLink">pressure gradient force
and, as we’ll see, higher wind speeds. Depending on the text you
read, you’ll see a number of different forms of the American
Meteorological Society, cited 2020: Pressure Gradient Force.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Pressure-gradient_force.]"
class="glossaryLink">pressure gradient force. Two equivalent
representations are given below.
The PGF as defined above is simply a change in pressure divided
by the distance and air density. This gives the force per unit
mass, hence PGF/m. Above we define PGF in the x-direction,
but it can act in any horizontal plane. The units of the PGF
will be in units of acceleration above because Force = Mass *
Acceleration, but because we’re dividing the left-hand side by
Mass the units will be m s-2. The negative signs in the above
equation is due to the fact that the PGF acts from high pressure
to low pressure.
ATMO 200 • 295
Another equivalent way to represent the PGF is as follows.
Because Mass = Volume * Density, we then see that Volume
= Mass/Density. The second equation is identical to the first,
except that Mass/Density is on the right hand side in the form of
Volume instead of being split between left and right sides of the
equals sign. Also, the variable L is used for length or distance
instead of Δx so that it isn’t for a specific direction. Similarly,
the absolute value of the PGF is calculated, and the direction is
determined, by the location of High and Low pressure systems.
Typically in class we won’t calculate the PGF, only the PG.
However, it’ll be widely used in force balance equations.
Advection
While American Meteorological Society, cited 2020:
Advection. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Advection.]"
class="glossaryLink">advection is included in the list of
forces, it is actually not a true force. Still, American
Meteorological Society, cited 2020: Advection. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Advection.]" class="glossaryLink">advection can result in
a change of wind speed in some locations. Wind moving through
a point carries specific momentum, which is defined as
momentum per unit mass. Recall that momentum is mass times
velocity. Specific momentum then is simply equal to the velocity
296 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
or wind speed. Therefore, as wind moves by a point, the wind
can move or advect variations in winds to the fixed location.
Centrifugal Force
The American Meteorological Society, cited 2020:
Centrifugal Force. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Centrifugal_force.]"
class="glossaryLink">centrifugal
force is an apparent force that includes the effects of inertia
for winds moving along a curved path. The directionality of
the American Meteorological Society, cited 2020: Centrifugal
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Centrifugal_force.]"
class="glossaryLink">centrifugal force points outward from the
center of the curve. The American Meteorological Society, cited
2020: Centrifugal Force. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Centrifugal_force.]"
class="glossaryLink">centrifugal force is the opposite of the
centripetal force. As we know, inertia is the physical tendency
to remain unchanged. Therefore inertia causes an American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel to “want” to
move along a straight line. Turning the American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel along a
curved path requires a centripetal force that pulls inward to the
ATMO 200 • 297
center of rotation. As a result, a net imbalance of other forces
occurs.
You have felt the American Meteorological Society, cited 2020:
Centrifugal Force. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Centrifugal_force.]"
class="glossaryLink">centrifugal force many times in your life.
The American Meteorological Society, cited 2020: Centrifugal
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Centrifugal_force.]"
class="glossaryLink">centrifugal force is easily felt as you
travel in a moving vehicle around a corner. The force that you
feel pulling you outwards is the American Meteorological
Society, cited 2020: Centrifugal Force. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Centrifugal_force.]" class="glossaryLink">centrifugal force.
Coriolis Force
The American Meteorological Society, cited 2020: Coriolis
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force (CF) is another apparent
force that occurs due to the rotation of Earth. The American
Meteorological Society, cited 2020: Coriolis Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Coriolis_force.]" class="glossaryLink">Coriolis force is a
deflecting force. It acts only on objects already in motion.
Therefore it cannot create wind, but it can change the wind
direction by deflecting it. The American Meteorological Society,
298 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
cited 2020: Coriolis Force. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force acts perpendicular to the
direction of motion, but whether the American Meteorological
Society, cited 2020: Coriolis Force. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Coriolis_force.]" class="glossaryLink">Coriolis force acts 90°
to the right or left of the motion vector depends on the
hemisphere on Earth. In the Northern Hemisphere, the American
Meteorological Society, cited 2020: Coriolis Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Coriolis_force.]" class="glossaryLink">Coriolis force
acts 90° to the right of the motion vector while in the Southern
Hemisphere, the force acts 90° to the left of the motion vector.
The equation below gives the American Meteorological Society,
cited 2020: Coriolis Force. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force
where m is for the mass of the object in kg, u is the speed of the
object in m s-1, the symbol Φ denotes latitude in degrees, and the
angular rotation rate, Ω, is found from the rotation rate of Earth.
Earth turns 2*pi radians over 24 hrs, so 2*pi/24 hrs gives Ω =
7.27E-5 radians·s–1. Based on this, we can see that the Coriolis
parameter will be 0 at the equator when sin(0)=0 and maximized
at the poles when sin(90)=1.
We can also see that the American Meteorological Society, cited
2020: Coriolis Force. Glossary of Meteorology. [Available
ATMO 200 • 299
online at http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force is strongly dependent on
the speed of the object. If we assume the “object” is actually
wind, stronger winds will be more strongly deflected by the
American Meteorological Society, cited 2020: Coriolis Force.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force.
Turbulent Drag
American Meteorological Society, cited 2020: Turbulent Drag.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Drag_law.]"
class="glossaryLink">Turbulent drag occurs when Earth’s
surface or objects on it cause resistance to airflow and reduce the
wind speed. Any object on Earth’s surface can cause drag, such
as grass, trees, and buildings, which block and decelerate wind.
The bottom layer of the troposphere around 0.3 to 3 km thick
is called the American Meteorological Society, cited 2019:
Atmospheric boundary layer. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Atmospheric_boundary_layer.]"
class="glossaryLink">atmospheric boundary layer (ABL).
Turbulence in the ABL mixes the extremely slow movement of
air near the surface with the faster movement of air in the ABL
and slows the wind speed in the entire ABL.
300 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Force Balances
The five forces from above affect aspects of horizontal wind
speed and direction, and result in a number of common force
balances found throughout Earth’s atmosphere.
Geostrophic Balance
American Meteorological Society, cited 2020: Geostrophic
Balance. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Geostrophic_balance.]"
class="glossaryLink">Geostrophic balance is arguably the most
important force balance in the atmosphere and holds nearly
all the time, except for a few specific cases scenarios to be
discussed later. When in American Meteorological Society, cited
2020: Geostrophic Balance. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Geostrophic_balance.]"
class="glossaryLink">geostrophic
balance, wind in the atmosphere has a balance between the
American Meteorological Society, cited 2020: Pressure Gradient
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Pressure-gradient_force.]"
class="glossaryLink">pressure gradient force and the American
Meteorological Society, cited 2020: Coriolis Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Coriolis_force.]" class="glossaryLink">Coriolis force. In
American Meteorological Society, cited 2020: Geostrophic
Balance. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Geostrophic_balance.]"
class="glossaryLink">geostrophic balance, PGF = CF. The
ATMO 200 • 301
resulting wind is called a geostrophic wind. Setting the equation
for CF and PGF equal to each other and solving for u gives the
following equation for Ugeos.
Because geostrophic winds are dependent on the pressure
gradient, geostrophic winds are faster when American
Meteorological Society, cited 2020: Isobars. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Isobars.]" class="glossaryLink">isobars are closely
spaced.
A number of assumptions are implicit to American
Meteorological Society, cited 2020: Geostrophic Balance.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Geostrophic_balance.]"
class="glossaryLink">geostrophic
balance.
American
Meteorological Society, cited 2020: Geostrophic Balance.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Geostrophic_balance.]"
class="glossaryLink">Geostrophic balance applies only under
the following conditions: large temporal (>12 hrs) and large
spatial (> a few km) scales; above the ABL when no surface
friction is acting on the air; winds are steadily moving in a
straight direction (no acceleration, negligible vertical velocity);
finally, because the American Meteorological Society, cited
2020: Coriolis Force. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Coriolis_force.]"
302 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">Coriolis force is important for the
balance, it cannot hold at the equator when the CF is 0. The
typical bounds are often given as >2° latitude.
The path of the geostrophic wind is parallel to the American
Meteorological Society, cited 2020: Isobars. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Isobars.]" class="glossaryLink">isobars. In the Northern
Hemisphere, the wind direction is parallel to the straight
American Meteorological Society, cited 2020: Isobars. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Isobars.]" class="glossaryLink">isobars with the low
pressure to the left side of wind. In the Southern Hemisphere,
the direction is parallel to the straight American Meteorological
Society, cited 2020: Isobars. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars with the low pressure to the
wind’s right. The image below shows the force balance present
in a geostrophic wind in the northern hemisphere.
ATMO 200 • 303
Geostrophic wind force diagram in the northern hemisphere (Image
Created by Shintaro Russell via Paint.net).
To get into American Meteorological Society, cited 2020:
Geostrophic Balance. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Geostrophic_balance.]"
class="glossaryLink">geostrophic
balance, moving air will undergo American Meteorological
Society, cited 2020: Geostrophic Adjustment. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Geostrophic_adjustment.]"
class="glossaryLink">geostrophic adjustment. First, air feels
the pressure field (PGF) and begins moving from high to low
pressure. Next, the American Meteorological Society, cited
2020: Coriolis Force. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force (CF) deflects the object’s
direction once it is in motion. Finally, the air finds itself in a
balance between the PGF and the CF moving parallel to the
304 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
American Meteorological Society, cited 2020: Isobars. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Isobars.]" class="glossaryLink">isobars instead of across
them.
Gradient Wind
This next force balance applies when air is not moving in a
straight line. Gradient winds are winds flowing along curved
American Meteorological Society, cited 2020: Isobars. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Isobars.]" class="glossaryLink">isobars. Winds typically
blow along American Meteorological Society, cited 2020:
Isobars. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars, even if they are curved, but a
different name is needed because the force balance includes
one more component. Compared to geostrophic winds, gradient
winds feature a balance between the American Meteorological
Society, cited 2020: Coriolis Force. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Coriolis_force.]" class="glossaryLink">Coriolis force, the
American Meteorological Society, cited 2020: Pressure Gradient
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Pressure-gradient_force.]"
class="glossaryLink">pressure gradient force, and the American
Meteorological Society, cited 2020: Centrifugal Force. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Centrifugal_force.]"
class="glossaryLink">centrifugal
force. The American Meteorological Society, cited 2020:
ATMO 200 • 305
Centrifugal Force. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Centrifugal_force.]"
class="glossaryLink">centrifugal force arises because the air
is flowing on a curved path. The American Meteorological
Society, cited 2020: Centrifugal Force. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Centrifugal_force.]" class="glossaryLink">centrifugal force
acts in the same direction as the American Meteorological
Society, cited 2020: Coriolis Force. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Coriolis_force.]" class="glossaryLink">coriolis force, opposite
the American Meteorological Society, cited 2020: Pressure
Gradient Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Pressure-gradient_force.]"
class="glossaryLink">pressure gradient force.
Gradient wind force diagram (Image Created by Shintaro Russell via
Paint.net).
306 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Atmospheric Boundary Layer
Balanced wind in the American Meteorological Society, cited
2019: Atmospheric boundary layer. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Atmospheric_boundary_layer.]"
class="glossaryLink">atmospheric boundary layer (ABL)
occurs when there is a balance between the American
Meteorological Society, cited 2020: Pressure Gradient Force.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Pressure-gradient_force.]"
class="glossaryLink">pressure gradient force, American
Meteorological Society, cited 2020: Coriolis Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Coriolis_force.]" class="glossaryLink">Coriolis force, and
the frictional drag force. Both American Meteorological Society,
cited 2020: Wind Shear. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear turbulence and convective
turbulence cause drag, which results in the ABL wind being
slower than geostrophic (subgeostrophic), and causes the wind
to cross American Meteorological Society, cited 2020: Isobars.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars toward the low pressure.
ATMO 200 • 307
Atmospheric boundary layer wind force diagram (Image created by
Shintaro Russell via Paint.net).
Again, the frictional drag force acts in the plane of motion
and slows down the wind speed. The American Meteorological
Society, cited 2020: Pressure Gradient Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Pressure-gradient_force.]" class="glossaryLink">pressure
gradient force doesn’t change, but because the wind speed is
slower, the American Meteorological Society, cited 2020:
Coriolis Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force is weaker. When that
happens the wind cannot balance the American Meteorological
Society, cited 2020: Pressure Gradient Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Pressure-gradient_force.]" class="glossaryLink">pressure
gradient force, it is pulled more by the American Meteorological
Society, cited 2020: Pressure Gradient Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
308 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
wiki/Pressure-gradient_force.]" class="glossaryLink">pressure
gradient force, and turns toward the low pressure.
Cyclostrophic Wind
American Meteorological Society, cited 2020: Cyclostrophic
wind. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Cyclostrophic_wind.]"
class="glossaryLink">Cyclostrophic wind occurs at smaller
cyclonic scales (at the mesoscale) such as tornadoes,
waterspouts, and even the center of a tropical cyclone. Because
the scale is small, the American Meteorological Society, cited
2020: Coriolis Force. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force does not play a role. When
a small cyclonic scale such as a tornado first forms, both
tangential winds and American Meteorological Society, cited
2020: Centrifugal Force. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Centrifugal_force.]"
class="glossaryLink">centrifugal force increase much faster
than the American Meteorological Society, cited 2020: Coriolis
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force due to the very strong
American Meteorological Society, cited 2020: Pressure Gradient
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Pressure-gradient_force.]"
class="glossaryLink">pressure gradient force. As a result,
American Meteorological Society, cited 2020: Centrifugal
Force. Glossary of Meteorology. [Available online at
ATMO 200 • 309
http://glossary.ametsoc.org/wiki/Centrifugal_force.]"
class="glossaryLink">centrifugal force balances with the
American Meteorological Society, cited 2020: Pressure Gradient
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Pressure-gradient_force.]"
class="glossaryLink">pressure gradient force, ignoring the
negligible effects of American Meteorological Society, cited
2020: Coriolis Force. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force. Because the scale is small
and independent of the American Meteorological Society, cited
2020: Coriolis Force. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force, the direction of
cyclostrophic winds can be either clockwise or counterclockwise
in both hemispheres. For anticyclones or highs, however, they do
not typically have strong pressure gradients. Thus, winds around
the high are too weak to be in cyclostrophic balance.
310 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Cyclostrophic wind force diagram where the pressure gradient force is
balanced by the centrifugal force (Image Created by Shintaro Russell via
Paint.net).
All of the wind balances discussed (American Meteorological
Society, cited 2020: Geostrophic Balance. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Geostrophic_balance.]" class="glossaryLink">geostrophic
balance, gradient wind, ABL wind, and American
Meteorological Society, cited 2020: Cyclostrophic wind.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Cyclostrophic_wind.]"
class="glossaryLink">cyclostrophic wind) occur in Earth’s
atmosphere under differing conditions. The following chapters
will make these applications clearer and you can check back here
for reference.
ATMO 200 • 311
Chapter 10: Questions to Consider
1.
Explain why wind occurs.
2.
What are u, v, and w?
3.
Which forces influence the direction
and speed of horizontal winds?
4.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=341
5.
Drag the terms to their correct
position:
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=341
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
312 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=341
Chapter 11: General Circulation
ALISON NUGENT AND DAVID DECOU
Learning Objectives
By the end of this chapter, you should be able to:
1.
Describe the differential heating Earth
experiences, and how heat is
redistributed
2.
Diagram vertical atmospheric
circulations (American Meteorological
Society, cited 2020: Hadley Cell.
Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/
wiki/Hadley_cell.]"
class="glossaryLink">Hadley cell,
313
314 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
American Meteorological Society, cited
2020: Ferrel cell. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Ferrel_cell.]"
class="glossaryLink">Ferrel cell,
American Meteorological Society, cited
2020: Polar Cell. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Polar_cell.]" class="glossaryLink">Polar
cell)
3.
Diagram surface wind directions
(American Meteorological Society, cited
2020: Trade Winds. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Trade_winds.]"
class="glossaryLink">trade winds, belt
of American Meteorological Society,
cited 2020: Westerlies. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Westerlies.]"
class="glossaryLink">westerlies, etc.)
4.
Discuss the distribution of heat over
Earth’s surface and how it drives global
circulation, including its connection to
the American Meteorological Society,
cited 2020: Coriolis Force. Glossary of
ATMO 200 • 315
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Coriolis_force.]"
class="glossaryLink">Coriolis force
Introduction
The focus of this chapter is on the typical wind circulations
found on Earth as a result of the forces affecting wind in the
atmosphere, as introduced in Chapter 10. The average global
winds are called the general circulation of the atmosphere. To
find typical wind circulations, one needs to average wind speed
and duration over a long period of time. Averaging over time
removes short duration fluctuations, allowing the primary sense
of movement to be visualized.
The reason we have global wind patterns is ultimately due to
a differentially heated, rotating Earth. The differential heating
of Earth continually causes an imbalance in air pressure and
temperature around the world, which in turn causes a continuous
general circulation of winds that attempt to restore balance.
While actual winds in a given place and time may differ from
the average general circulation, the average can provide an
explanation for how and why the winds prevail from a particular
direction in a certain place. For example, the prevailing surface
winds tend to be westerly in the continental United States, but
northeasterly in Hawai’i. The general circulation also serves as
316 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
a model for how heat and momentum are transported from the
equator to the poles.
Differential Heating
Because the Earth is round, solar American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation is not equally
spread at all latitudes. Near the equator where sunlight shines
directly on Earth, more solar American Meteorological Society,
cited 2019: Radiation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation per square meter is received as
compared to near the poles where sunlight shines at sharp angles
to the surface (see image below). Toward Earth’s poles, the same
solar American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation is spread over a larger surface
area such that each square meter of Earth’s surface gets less
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation at the poles. As Earth rotates,
the incoming solar American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
ATMO 200 • 317
class="glossaryLink">radiation is zonally spread along latitude
lines.
Earth’s uneven heating by the sun due to the curvature of its
surface NASA (Public Domain).
In this way, incoming solar American Meteorological Society,
cited 2019: Radiation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation depends on latitude. The sun
shines more directly on tropical regions at lower latitudes than at
higher latitudes all year-round. Solar American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation adds heat to the
Earth-atmosphere-ocean system, and thus lower latitudes get
heated more than higher latitudes. This should be as expected
because we know the tropics are warmer than the polar regions.
While Earth is continually heated by the sun, it is also
continually losing American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
318 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy by emitting outgoing longwave
infrared (IR) American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation at all latitudes, and at all times,
both on the light and the dark side of the globe. You’ll recall
from Chapter 2 that IR American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation is emitted by Earth according
to the Stefan-Boltzman law, which is highly dependent on
temperature.
When averaged over the globe and over long time scales,
incoming UV American Meteorological Society, cited 2019:
Radiation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation exactly balances outgoing IR
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation. But, latitude by latitude,
incoming UV and outgoing do not perfectly balance. More solar
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy is received by the
Earth in the tropics, and while the cooling by outgoing IR
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
ATMO 200 • 319
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation helps to offset this, there is still
a net gain of radiative American Meteorological Society, cited
2019: Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy in the tropics. However, near
Earth’s poles, incoming solar American Meteorological Society,
cited 2019: Radiation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation is less direct and too weak to
offset the cooling by outgoing IR American Meteorological
Society, cited 2019: Radiation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Radiation.]" class="glossaryLink">radiation, so there is net
cooling at the poles. This causes warmer air at the equator, and
cold air at the poles and drives Earth’s atmospheric general
circulation.
320 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Incoming solar radiation (yellow dashed) and outgoing infrared
radiation (red solid) diagram (CC BY-NC-SA 4.0).
The above image illustrates this more directly. Incoming solar
American Meteorological Society, cited 2019: Radiation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation is focused near the equator,
while outgoing IR American Meteorological Society, cited
2019: Radiation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Radiation.]"
class="glossaryLink">radiation is relatively evenly spread
across all latitudes. This results in the below American
Meteorological Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy surplus near the
equator and deficit toward the poles.
ATMO 200 • 321
Heat transport balances the net radiative imbalances (CC BY-NC-SA
4.0).
Earth’s general circulation attempts to redistribute heat around
the globe and rebalance the American Meteorological Society,
cited 2019: Energy. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy imbalances inherent in an
unevenly heated, rotating planet. However, the general
circulation cannot instantly balance global temperature,
especially when the uneven heating is continuous. Therefore, a
meridional temperature gradient always remains.
In an attempt to balance out Earth’s incoming and outgoing
American Meteorological Society, cited 2019: Energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy, warm air is
322 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
transported toward the poles, while cool air flows back toward
the equator. This seems simple enough. However, this seemingly
simple flow is complicated by many factors, including Earth’s
rotation, the position of continents, interactions with the oceans
and many others. In order to build an understanding of this
process, we’ll start with some simplified models and build
complexity.
Single-Cell Model
The first model we’ll examine is the single-cell model. With this
model, we make the following assumptions.
1. The earth is entirely covered with water. This is to
remove any land-sea interactions.
2. There are no seasons and the sun is always shining
directly over the equator. This removes seasonal wind
shifts.
3. There is no American Meteorological Society, cited
2020: Coriolis Force. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Coriolis_force.]" class="glossaryLink">Coriolis
force. While the Earth rotates to spread heat along
latitudinal lines, this allows us to only be concerned
with the American Meteorological Society, cited
2020: Pressure Gradient Force. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Pressuregradient_force.]" class="glossaryLink">pressure
ATMO 200 • 323
gradient force.
With these assumptions in place, Earth’s global circulation
would like the figure below, with one giant vertically
overturning cell in each hemisphere. The excess heating at the
equator is transported poleward by rising warm air, which is
replaced by cold sinking polar air moving equatorward. This
circulation is known as the American Meteorological Society,
cited 2020: Hadley Cell. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Hadley_cell.]"
class="glossaryLink">Hadley
cell.
The
American
Meteorological Society, cited 2020: Hadley Cell. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Hadley_cell.]" class="glossaryLink">Hadley cell is known
as a thermally direct circulation because in it, warm air is rising
and cold air is sinking.
324 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
The single-cell model of Hadley cells on a planet (CC BY-SA 4.0).
The circulation can be thought of in two ways. In the first,
hot air at the equator rises because it is warm and buoyant. It
reaches the tropopause, spreading laterally north and south at
high elevations. To compensate for the rising air, surface air
flows toward the equator, resulting in convergence and further
uplift. Continuity of this circulation results in a global
circulation with rising air at the equator and sinking air at the
poles.
A second way to view global circulation is that the excess
heating of air at the equator creates a large area of low pressure
at the surface of the planet, while excess cooling at the poles
creates high pressure at the surface. This global horizontal
ATMO 200 • 325
pressure gradient causes air to flow from high to low at the
surface (pole to equator), where the air subsequently rises at the
equator and flows back to the poles and sinks.
Both reasonings are plausible, its a matter of whether you focus
on temperature or pressure. The temperature differences and the
resulting pressure differences are intertwined and both important
for the general circulation.
While this single-cell model can explain some phenomenon and
works in some ways (and on some planetary bodies), it is not
the reality on Earth. Earth is a rotating planet, so we need
to consider the American Meteorological Society, cited 2020:
Coriolis Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force in addition to the American
Meteorological Society, cited 2020: Pressure Gradient Force.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Pressure-gradient_force.]"
class="glossaryLink">pressure gradient force. In the single-cell
model, as upper level air flows from the equator toward the
poles, it would be deflected by the American Meteorological
Society, cited 2020: Coriolis Force. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Coriolis_force.]" class="glossaryLink">Coriolis force. In the
northern hemisphere, for example, this deflection would be
toward the right resulting in a wind from west to east at upper
levels. In this way, the air moving from the equator to the poles
would never make it there because of the rotation of Earth. A
different model is needed.
326 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Three-Cell Model
If we allow for the effects of a rotating planet, the simple singlecell model above breaks down into multiple cells in each
hemisphere as shown in the figure below. It may look more
complex and unrelated to the single-cell model, but there are
many similarities from above. There is still excess heating in
equatorial regions and excess cooling in polar regions. Instead
of heat being redistributed by one massive American
Meteorological Society, cited 2020: Hadley Cell. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Hadley_cell.]" class="glossaryLink">Hadley cell from the
equator to the poles, there are now three convective cells. The
first of these is still the same thermally direct American
Meteorological Society, cited 2020: Hadley Cell. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Hadley_cell.]" class="glossaryLink">Hadley cell from
before, but now it extends only from the equator to about 30°
latitude. The poles still have a large high pressure system, while
the equator has a large belt of low pressure along it. Let’s take
a closer look at what happens to the rising air just above the
equator.
ATMO 200 • 327
Three-cell model of the rotating Earth and the resulting wind
circulations (CC BY-SA 4.0).
At the equator, the air near the surface is warm, winds are light,
and the pressure gradient is weak. This region of monotonous
American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather is known as the American
Meteorological Society, cited 2020: Doldrums. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Doldrums.]"
class="glossaryLink">doldrums. The warm air here rises,
condensing into massive cumulonimbus clouds and
328 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
thunderstorms, which release large amounts of American
Meteorological Society, cited 2019: Latent heat. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Latent_heat.]" class="glossaryLink">latent heat as they
form. The additional heat makes the air even more likely to
rise, and provides the American Meteorological Society, cited
2019: Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy that drives the rising branch of
the American Meteorological Society, cited 2020: Hadley Cell.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Hadley_cell.]"
class="glossaryLink">Hadley cell. This rising air reaches the
stable tropopause, which blocks it from rising further, causing
the air to diverge at upper levels and move poleward. Due to the
American Meteorological Society, cited 2020: Coriolis Force.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force, this upper level poleward
flow is deflected to the right in the Northern Hemisphere and
to the left in the Southern Hemisphere, providing American
Meteorological Society, cited 2020: Westerlies. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Westerlies.]" class="glossaryLink">westerlies aloft (near
the tropopause) in both hemispheres in the American
Meteorological Society, cited 2020: Hadley Cell. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Hadley_cell.]" class="glossaryLink">Hadley cell.
As air moves poleward from equatorial regions, it is constantly
ATMO 200 • 329
experiencing radiational cooling as it emits infrared American
Meteorological Society, cited 2019: Radiation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Radiation.]"
class="glossaryLink">radiation.
Simultaneously, this air begins to converge and pile up as it
approaches the mid-latitudes (around 30° latitude in both
hemispheres). This convergence of air far above the surface
increases the mass of air aloft, increasing the pressure at the
surface. This increase in surface pressure results in a belt of
high pressure centers called American Meteorological Society,
cited 2020: Subtropical Highs. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Subtropical_high.]"
class="glossaryLink">subtropical
highs around 30°N and 30°S. These latitudes are commonly
known as the horse latitudes.
As this converging air above the American Meteorological
Society, cited 2020: Subtropical Highs. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Subtropical_high.]"
class="glossaryLink">subtropical
highs slowly descends, it warms adiabatically by compression.
This sinking air, dries the atmosphere creating generally clear
skies and little rain. Over the oceans, weak pressure gradients
in the high centers produce weak winds. Some of these lighter
surface winds begin to move back toward the equator, and are
deflected by the American Meteorological Society, cited 2020:
Coriolis Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force. This causes northeasterly
winds in the Northern Hemisphere and southeasterly winds in
330 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
the Southern Hemisphere in tropical regions. These winds are
known as the American Meteorological Society, cited 2020:
Trade Winds. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Trade_winds.]"
class="glossaryLink">trade winds, and they have a strong
influence over the daily wind patterns in Hawai’i. Near the
equator, the northeasterly and southeasterly American
Meteorological Society, cited 2020: Trade Winds. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Trade_winds.]"
class="glossaryLink">trade
winds
converge at the surface at what is known as the American
Meteorological
Society,
cited
2020:
Intertropical
Convergence Zone. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Intertropical_convergence_zone.]"
class="glossaryLink">intertropical
convergence
zone
(ITCZ). Here, convergence further reinforces the rising branch
of the American Meteorological Society, cited 2020: Hadley
Cell. Glossary of Meteorology. [Available
http://glossary.ametsoc.org/wiki/Hadley_cell.]"
class="glossaryLink">Hadley cell.
online
at
Back at 30° latitude, while some of the air sinking along the
American Meteorological Society, cited 2020: Subtropical
Highs. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Subtropical_high.]"
class="glossaryLink">subtropical highs goes equatorward to
complete the American Meteorological Society, cited 2020:
Hadley Cell. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Hadley_cell.]"
ATMO 200 • 331
class="glossaryLink">Hadley cell, some sinking air also moves
poleward. This poleward moving surface air travels from from
30° to 60° and is again deflected by the American
Meteorological Society, cited 2020: Coriolis Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Coriolis_force.]" class="glossaryLink">Coriolis force.
This results in the prevailing surface American Meteorological
Society, cited 2020: Westerlies. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Westerlies.]" class="glossaryLink">westerlies that impact
the mid-latitudes in both hemispheres. It is for this reason that
American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather moves west to east across the
continental US. Often, this westerly flow is interrupted by high
and low pressure systems that move with the mean surface
flow. We’ll learn more about this in the next two chapters. As
the surface air travels poleward from 30° to 60°, it collides
with cold polar air moving equatorward. These air masses do
not mix easily, and are separated by a boundary known as the
American Meteorological Society, cited 2020: Polar Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front. At the American
Meteorological Society, cited 2020: Polar Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Polar_front.]" class="glossaryLink">polar front, surface
air converges and rises at the American Meteorological
Society, cited 2020: Subpolar Low. Glossary of Meteorology.
332 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
[Available online at http://glossary.ametsoc.org/wiki/
Subpolar_low_pressure_belt]"
class="glossaryLink">subpolar low, and storms and
convection develop here. Some of this rising air goes all the
way up to the tropopause where it moves back to 30° latitude
and sinks at the subtropical high along with the descending
branch of the American Meteorological Society, cited 2020:
Hadley Cell. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Hadley_cell.]"
class="glossaryLink">Hadley cell. This circulation cell from
30° to 60° is known as the American Meteorological Society,
cited 2020: Ferrel cell. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Ferrel_cell.]"
class="glossaryLink">Ferrel cell, which is a thermally
indirect circulation in which cool air rises and warm air sinks.
Behind the American Meteorological Society, cited 2020: Polar
Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front in the Northern hemisphere,
cold surface polar air moves from the poles toward 60°. As the
air moves equatorward, it is again deflected by the American
Meteorological Society, cited 2020: Coriolis Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Coriolis_force.]" class="glossaryLink">Coriolis force. In
the Arctic regions, air typically flows from the northeast while in
the Antarctic, air flows from the southeast. These are known
as the polar easterlies. Along the American Meteorological
Society, cited 2020: Polar Front. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
ATMO 200 • 333
Polar_front.]" class="glossaryLink">polar front where cold
polar air collides with warm air from the American
Meteorological Society, cited 2020: Ferrel cell. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Ferrel_cell.]" class="glossaryLink">Ferrel cell, some of
the rising air moves back toward the poles, which gets deflected
as a westerly wind aloft. Eventually this air reaches the poles,
sinks back to the surface, and flows back toward the American
Meteorological Society, cited 2020: Polar Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Polar_front.]" class="glossaryLink">polar front, which
gives us the American Meteorological Society, cited 2020:
Polar Cell. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Polar_cell.]"
class="glossaryLink">Polar cell.
To summarize, looking back at the three-cell model picture:
there are two major belts of high pressure and two major belts
of low pressure in each hemisphere (if you include the equator
in both). Areas of high pressure and sinking air exist near 30°
latitude and at the poles. Regions of low pressure and rising air
exist over the equator and near 60° latitude by the American
Meteorological Society, cited 2020: Polar Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Polar_front.]" class="glossaryLink">polar front. By
knowing that winds travel counterclockwise (clockwise) around
low pressure systems in the Northern Hemisphere (Southern
Hemisphere), and clockwise (counterclockwise) around high
pressure systems in the Northern Hemisphere (Southern
Hemisphere), you can get a pretty general idea of how surface
334 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
winds blow around the world on average. American
Meteorological Society, cited 2020: Trade Winds. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Trade_winds.]" class="glossaryLink">Trade winds blow
from the American Meteorological Society, cited 2020:
Subtropical Highs. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Subtropical_high.]"
class="glossaryLink">subtropical highs at 30° to the equator,
the American Meteorological Society, cited 2020: Westerlies.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Westerlies.]"
class="glossaryLink">westerlies blow from the American
Meteorological Society, cited 2020: Subtropical Highs. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Subtropical_high.]"
class="glossaryLink">subtropical
highs to the American Meteorological Society, cited 2020: Polar
Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front, and the polar easterlies blow
from the poles to the American Meteorological Society, cited
2020: Polar Front. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front at the surface. Areas where
these winds converge will have rising motion and low pressure
at the surface, and regions where these winds diverge will have
sinking motion and high pressure at the surface.
How does this three-cell model match with reality? While some
minor discrepancies exist, for example in reality much of the
upper-level winds in the mid-latitudes are westerly like the
ATMO 200 • 335
surface, while the American Meteorological Society, cited 2020:
Ferrel cell. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Ferrel_cell.]"
class="glossaryLink">Ferrel cell suggests there should be
easterly winds aloft. However, this model is roughly accurate for
surface winds and provides a really good first order pattern for
general circulation.
Average near surface winds (CC BY-NC-SA 4.0).
How does the real world’s average surface sea level pressure
336 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
field compare with the above picture? When we add in the
continents, ice masses, oceans, mountains, and forest, we get
an average that looks something like the below two figures.
The following maps show the mean sea-level pressure field for
January and July, averaged from 1981 to 2010.
Looking at the two maps below, you may notice that there are
some areas where low and high pressure systems seem to persist
throughout the year – these are known as semipermanent highs
and semipermanent lows. These include the Bermuda-Azores
High, the Pacific High, the Icelandic Low, and the Aleutian
Low.
Map of semipermanent highs and lows during the month of January (CC
BY-NC-SA 4.0).
ATMO 200 • 337
Map of semipermanent highs and lows during the month of July (CC
BY-NC-SA 4.0).
Global Surface Winds
The following graphic nicely illustrates all of the above
phenomenon for global surface winds. The polar easterlies, midlatitude American Meteorological Society, cited 2020:
Westerlies. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Westerlies.]"
class="glossaryLink">westerlies, and tropical American
Meteorological Society, cited 2020: Trade Winds. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Trade_winds.]" class="glossaryLink">trade winds are all
visible. Now with the continents and land masses added, we are
able to see where on Earth these surface winds are observed.
338 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Circulation of Earth’s atmosphere (CC BY-SA 3.0).
Jet streams
There is one final important piece of general circulation that
deserves a discussion, and that is jet streams. In the below image
you can see two jet streams: the subtropical jet and the polar
jet. The figure shows the average position of the jet streams in
the Northern Hemisphere in the winter, as well as their relation
to the tropopause. The figure shows jet streams flowing from
west to east. We can see that there are two jets located right
under the tropopause.
The subtropical American
Meteorological Society, cited 2020: Jet Steam. Glossary of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Jet_stream.]"
ATMO 200 • 339
class="glossaryLink">jet stream is located near 30° latitude
about 13 km up, above the tropical high. The American
Meteorological Society, cited 2020: Polar Jet Stream.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Polar-front_jet_stream.]"
class="glossaryLink">polar jet stream is located near the
American Meteorological Society, cited 2020: Polar Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front about 10 km up, near 50° to
60° latitude. The difference in height of these jets is due to
their location at the tropopause, and the fact that the tropopause
is found higher in tropical regions than in polar regions due
to average layer temperature differences of the troposphere
underneath. The troposphere is thinner in polar regions than in
tropical regions due to colder, denser air at the poles.
Cross-section of the northern hemisphere circulation, and the positions
of the polar and subtropical jet streams (Public Domain).
If the general circulation of the atmosphere is like a giant
meandering river of air around the globe, then jet streams are
swiftly flowing currents within that river. Jet streams are
340 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
thousands of kilometers in length, and hundreds of kilometers in
width. In the core of a American Meteorological Society, cited
2020: Jet Steam. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Jet_stream.]"
class="glossaryLink">jet stream (called a jet streak), wind
speeds are often higher than 100 knots and are occasionally
higher than 200 knots. The polar jet can sometimes merge with
the subtropical jet if it sweeps southward enough, and it
occasionally splits into two jet streams.
ATMO 200 • 341
The position of the polar jet stream and the subtropical jet stream (CC
BY-NC-SA 4.0).
The existence of jet streams is ultimately due to the American
Meteorological Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]"
class="glossaryLink">energy
imbalance
between tropical and polar regions. How exactly do they
form? As mentioned before, the American Meteorological
Society, cited 2020: Polar Front. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
342 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Polar_front.]" class="glossaryLink">polar front is a boundary
between colder polar air and warmer subtropical air. Because of
this, the strongest temperature gradient occurs along the polar
frontal zone. This rapid change of temperature with distance also
causes a rapid pressure change, due to the thermal wind effect
(a vertical shear in the geostrophic wind caused by a horizontal
temperature gradient). This strong pressure gradient across the
American Meteorological Society, cited 2020: Polar Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front causes intense wind speeds
that become the American Meteorological Society, cited 2020:
Jet Steam. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Jet_stream.]"
class="glossaryLink">jet stream. The temperature contrast
between north and south along the American Meteorological
Society, cited 2020: Polar Front. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Polar_front.]" class="glossaryLink">polar front is more intense
during the winter than during the summer, so the polar jet is also
stronger during the winter. During winter, the leading edge of
the cold polar air pushes further south into subtropical areas.
During the summer, the American Meteorological Society, cited
2020: Polar Front. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front retreats into higher latitudes
and is weakened.
The subtropical American Meteorological Society, cited 2020:
Jet Steam. Glossary of Meteorology. [Available online at
ATMO 200 • 343
http://glossary.ametsoc.org/wiki/Jet_stream.]"
class="glossaryLink">jet stream tends to form just above the
descending branch of the American Meteorological Society,
cited 2020: Hadley Cell. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Hadley_cell.]"
class="glossaryLink">Hadley cell, at about 12 km altitude.
Here, a boundary exists between warmer equatorial air and
cooler air that has been cycled up and around the American
Meteorological Society, cited 2020: Ferrel cell. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Ferrel_cell.]" class="glossaryLink">Ferrel cell from the
American Meteorological Society, cited 2020: Polar Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front. This is sometimes referred
to as the subtropical American Meteorological Society, cited
2020: Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front, but it does not extend all the way to
the surface. Here, the temperature gradient is strongest aloft near
the tropopause, which induces a sharp pressure gradient and
strong winds aloft as well.
One final thought for Chapter 11: isn’t it fascinating that all of
these global winds are caused by differential heating and the
rotation of Earth? Everything is connected in one way or another
and fits together like a puzzle.
344 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Chapter 11: Questions to Consider
1.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=856
2.
What assumptions are made for a
single-cell model of Earth’s atmosphere?
3.
Drag the terms to their correct
position:
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=856
4.
Where are jet streams located? Why
do they differ in height?
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
ATMO 200 • 345
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=856
Chapter 12: Fronts and
Airmasses
ALISON NUGENT AND SHINTARO RUSSELL
Learning Objectives
By the end of this chapter, you should be able to:
1.
Describe what an “air mass” is and
how it forms
2.
Name some of the main types of air
masses
3.
Draw a vertical cross-section
schematic of a warm and cold American
Meteorological Society, cited 2020:
Front. Glossary of Meteorology.
346
ATMO 200 • 347
[Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front
4.
Recognize the symbols for warm, cold,
and occluded fronts
5.
Discuss the types of cloud patterns
associated with warm and cold fronts
6.
Explain how differences in
temperature (warm or cold fronts) or
differences in humidity (dry lines) are
important to American Meteorological
Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Weather.]"
class="glossaryLink">weather
Introduction
We’ve already learned that areas of low and high pressure can
be identified based on the American Meteorological Society,
cited 2020: Isobars. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Isobars.]"
class="glossaryLink">isobars on a American Meteorological
Society, cited 2019: Weather. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather map. We’ve also learned that
some regions on Earth typically have low pressure and while
348 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
others typically have high pressure. In this chapter we’ll learn
that some regions of the atmosphere have similar air properties
and are named by those properties as a collective mass of air or
“air mass”. High pressure systems, especially, are very common
air masses.
Air Masses
An air mass is an extensive body of air featuring generally
similar temperature and moisture characteristics. They can
extend thousands of square kilometers. Air mass are identified
based on their temperature and humidity characteristics as well
as their geographical region of origin. For example, if an air
mass is dry and warm and originated from the tropical region
over a continent, it would be called a continental Tropical (cT)
air mass. If an air mass is humid and cold and originated over
the ocean in the high latitudes, it would be called a maritime
Polar (mP) air mass. The air mass name will always begin with
a lower-case letter signifying either continental (c) or maritime
(m) and a second capital letter signifying Equatorial (E),
Tropical (T), Polar (P), Arctic (A), or Antarctic (AA).
Maritime air masses are humid air masses originating from
oceans or large bodies of water. Continental air masses are dry
air masses originating from land. Equatorial air masses are warm
moist air masses originating from the equatorial region. Tropical
air masses are warm air masses originating from the lower
latitudes. Polar air masses are cold air masses originating from
the upper latitudes. Arctic air masses are composed of extremely
cold air that originated from the poles. Arctic air masses are even
ATMO 200 • 349
colder and drier than Polar air masses, and Antarctic air masses
are even more extreme than Arctic air masses.
The common types of air masses are maritime Tropical (mT),
maritime Polar (mP), continental Polar (cP), continental
Tropical (cT), and continental Arctic (cA).
• Maritime Tropical (mT) air masses are warm, humid
air masses originating from the oceans in the tropics.
• Maritime Polar (mP) air masses are cold, humid air
masses originating from the oceans in the polar
latitudes.
• Continental Polar (cP) air masses are cold, dry air
masses originating from land regions in the polar
latitudes.
• Continental Tropical (cT) air masses are hot, dry air
masses originating from land in the tropics.
• Continental Arctic (cA) air masses are cold, dry air
masses originating from the North Pole.
• Continental Antarctic (cAA) air masses are extremely
cold and dry air masses originating from land at the
South Pole.
See the below image to visualize where these different types of
air masses typically originate.
350 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Air masses and their location of origin (Public Domain).
Creation of Air Masses
Air masses develop when air is present over a surface for an
extended period of time. This typically occurs in a high pressure
system with light wind. The areas where air masses develop are
called source regions. Air masses over warm surfaces usually
develop faster than those over colder surfaces because there is
weaker turbulence in the stable air over the cold surface. When
air masses shift from their source regions, they change over time
due to the surfaces and terrain over which the air masses flow.
Movement of Air Masses
Air masses do not remain over their source regions permanently.
Slight changes in American Meteorological Society, cited 2019:
Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather patterns may shift the air mass to
a new location. As air masses move, two things can occur.
First, as the air shifts over the different surface characteristics,
ATMO 200 • 351
the air mass begins changing. This process is called American
Meteorological Society, cited 2020: Air Mass Modification.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Airmass_modification.]"
class="glossaryLink">air mass modification. For example,
an mP airmass that moves from the Pacific ocean over the
mountains in the western continental US will typically dry as it
crosses the mountains, rains out its moisture, and warms over the
land surface until it becomes a cT airmass. The second thing that
happens when air masses move is that they may collide with
other air masses. When a collision occurs, the two air masses
develop a boundary called a American Meteorological Society,
cited 2020: Front. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front.
Surface Fronts
Surface fronts are the boundaries or transition zones between air
masses at the Earth’s surface. Changes in temperature, humidity,
wind, pressure, visibility, as well as particular cloud and
American Meteorological Society, cited 2020: Precipitation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation patterns are often observed
at fronts. There are four main types of fronts.
1. Cold fronts
2. Warm fronts
352 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
3. Occluded fronts
4. Stationary fronts
Fronts are named based on the characteristics of the air mass
that is replacing the prior air mass. For example, if a cold air
mass is moving toward a warm air mass, the boundary between
them will be called a cold American Meteorological Society,
cited 2020: Front. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front because the cold air is effectively
replacing the warm air from the perspective of a stationary point
on Earth’s surface.
Fronts are usually associated with low pressure systems. Frontal
boundaries on a map are labeled in the location where the
temperature gradient of the American Meteorological Society,
cited 2020: Front. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front meets the Earth’s surface.
ATMO 200 • 353
Cold Fronts
Diagram showing a vertical cross section through a cold front (CC
BY-SA 4.0).
Cold fronts are a transition zone in which the advancing cold
air mass replaces a retreating warmer air mass. On American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather maps, cold
fronts are drawn as blue lines with blue triangles pointing toward
the warmer air mass, in the direction of frontal movement, as
seen in the above image.
Because warm air is less dense than colder air, the cold air
stays on the bottom and warm air is forced to rise above the
advancing cold air. This forced lifting results in typical cloud
patterns ahead of a cold American Meteorological Society, cited
2020: Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front, which include cirrus and
354 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
cirrostratus clouds. The number of cumulus clouds in the warm
air mass increases as the frontal boundary approaches. Because
the warm air mass is forced to rise, atmospheric American
Meteorological Society, cited 2020: Instability. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Instability.]" class="glossaryLink">instability occurs along
the cold American Meteorological Society, cited 2020: Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front and results in towering cumulus and
cumulonimbus clouds, which may produce heavy rain and
thunderstorms along the frontal boundary. During the passage
of a cold American Meteorological Society, cited 2020: Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front, the wind direction generally shifts
from south or southwest (in the warm sector) to west or
northwest (in the cold sector) in the northern hemisphere. After
a cold American Meteorological Society, cited 2020: Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front’s
passage,
fair
American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather returns with the
appearance of cumulus and stratocumulus clouds.
ATMO 200 • 355
Warm Fronts
Diagram showing a vertical cross section through a warm front (CC
BY-SA 3.0).
Warm fronts are a transition zone in which the advancing warm
air mass replaces a retreating colder air mass. On American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather maps, warm
fronts are drawn as red lines with red semicircles pointing
toward the colder air mass in the direction of the frontal
movement.
Advancing warm air is forced to rise above the retreating cold
dense air. Again, because of this forced lifting, typical cloud
patterns are common ahead of a American Meteorological
Society, cited 2020: Warm Front. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Warm_front.]" class="glossaryLink">warm front. These include
upper-level clouds such as cirrus and cirrostratus clouds before
356 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
clouds thicken and lower-level clouds like altostratus,
nimbostratus, and fog near the frontal boundary. Because the
air mass is rising along the American Meteorological Society,
cited 2020: Warm Front. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Warm_front.]"
class="glossaryLink">warm front, clouds form and steady
American Meteorological Society, cited 2020: Precipitation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation may occur. During the
passage of a American Meteorological Society, cited 2020:
Warm Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Warm_front.]"
class="glossaryLink">warm front, wind direction shifts from
east or southeast (in the cold sector) to southwest (in the warm
sector) in the northern hemisphere. After a American
Meteorological Society, cited 2020: Warm Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Warm_front.]"
class="glossaryLink">warm
front’s
passage, cloud cover and American Meteorological Society,
cited 2020: Precipitation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation decreases with only
scattered cumulus clouds remaining.
ATMO 200 • 357
Occluded Fronts
Diagram showing a vertical cross section through an occluded front
(Public Domain).
Occluded fronts are a frontal boundary that forms when a cold
American Meteorological Society, cited 2020: Front. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Front.]" class="glossaryLink">front catches up to a
American Meteorological Society, cited 2020: Warm Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Warm_front.]"
class="glossaryLink">warm front. Cold fronts move faster than
warm fronts, so cold fronts can sometimes catch up to warm
fronts, but not the other way around.
There are two types of occlusions: cold occlusions and warm
occlusions. Cold occlusions (shown above) occur when the
advancing air mass is colder than the retreating air mass. Warm
occlusions occur when the advancing air mass is warmer than
the retreating air mass. On American Meteorological Society,
cited 2019: Weather. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Weather.]"
358 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">weather maps, occluded fronts are drawn
as purple lines with purple triangles and purple semi circles.
You’ll notice that this symbol is a combination of the cold
American Meteorological Society, cited 2020: Front. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Front.]" class="glossaryLink">front and American
Meteorological Society, cited 2020: Warm Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Warm_front.]" class="glossaryLink">warm front symbols,
which isn’t surprising because an American Meteorological
Society, cited 2020: Occluded Front. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Occluded_fronts.]" class="glossaryLink">occluded front is
effectively a combination of the two.
Frontal symbol for an occluded frontal boundary (Public
Domain).
Occluded fronts are the final stage of a frontal boundary because
warm air above cool air is a stable scenario.
Stationary Fronts
Stationary fronts are a type of frontal system that are almost
stationary with the winds flowing nearly parallel and from the
opposite paths in each side separated by the American
Meteorological Society, cited 2020: Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
ATMO 200 • 359
wiki/Front.]" class="glossaryLink">front. On American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather maps, stationary
fronts are drawn as alternating blue and red lines with blue
triangles pointing toward the warmer air mass and red
semicircles pointing toward the colder air mass. This is the only
scenario where the direction of the symbols does not indicate a
direction of movement.
Frontal symbol for a stationary frontal boundary (Public
Domain).
Other Frontal-like Features
Not all American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather
features
have
frontal
characteristics, such as a trough and a dryline. While troughs
feature a change in cloud and American Meteorological Society,
cited 2020: Precipitation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation pattern, it lacks the sharp
change in temperature and moisture as observed in fronts.
Drylines are a boundary between warm, moist air and warm, dry
air. Because both air masses are warm, a dryline cannot be
360 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
classified as either a American Meteorological Society, cited
2020: Warm Front. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Warm_front.]"
class="glossaryLink">warm front or a cold American
Meteorological Society, cited 2020: Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Front.]" class="glossaryLink">front. Drylines commonly
occur during spring and summer in southwestern United States,
particularly in Texas. Warm, humid air from the Gulf of Mexico
meets with the warm, dry air from the desert plateau. During
the afternoon, convective clouds and even thunderstorms can
develop in drylines as moist air rises over the denser dry air.
Chapter 12: Questions to Consider
1.
What is an air mass? What are some
different types of air masses and where
do they originate?
2.
Label the cold American
Meteorological Society, cited 2020:
Front. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front crosssection:
ATMO 200 • 361
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=379
3.
Label the American Meteorological
Society, cited 2020: Warm Front.
Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/
wiki/Warm_front.]"
class="glossaryLink">warm front crosssection:
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=379
4.
Match the American Meteorological
Society, cited 2020: Front. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front name with
the correct symbology:
An interactive or media element has
362 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=379
5.
What cloud and American
Meteorological Society, cited 2020:
Precipitation. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Precipitation.]"
class="glossaryLink">precipitation
patterns are typically associated with
warm and cold frontal passage?
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=379
Chapter 13: Extratropical
Cyclones
ALISON NUGENT AND DAVID DECOU
Learning Objectives
By the end of this chapter, you should be able to:
1.
Describe how American
Meteorological Society, cited 2020:
Cyclogenesis. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Cyclogenesis.]"
class="glossaryLink">cyclogenesis
occurs
363
364 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
2.
Identify areas on a map where midlatitude cyclones are common, and
explain why they move where they do
3.
Sketch the frontal systems involved in
a mid-latitude cyclone
4.
Understand the hazards associated
with mid-latitude cyclones
5.
Discuss the relationship between sea
level pressure, high and low pressure
systems, air columns and mass budgets
as a closed system
Satellite image of a mid-latitude cyclone over North America (Public
Domain).
ATMO 200 • 365
Introduction
For well over a century, forecasters have been aware that areas of
falling barometric pressures are often accompanied by American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation and
strong winds. However, it wasn’t until the early 1900’s that
atmospheric scientists began piecing together a more complete
picture of how low pressure systems develop, as well as the
American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather associated with them.
Recall that a cyclone is an area of low pressure, around which
winds blow counterclockwise in the Northern Hemisphere and
clockwise in the Southern Hemisphere. This is due to the fact
that winds blow from high to low pressure, but are deflected
by the American Meteorological Society, cited 2020: Coriolis
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Coriolis_force.]"
class="glossaryLink">Coriolis force (perpendicular to the right
of the motion vector in the Northern Hemisphere, left in the
Southern Hemisphere). The focus of this chapter is cyclonic
storm systems that form in the mid-to-high latitudes outside of
the tropics. These storm systems are either called mid-latitude
frontal cyclones, extratropical cyclones, wave cyclones, or
simply frontal cyclones. Tropical cyclones will be the focus of a
later chapter.
366 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Shortly after World War I, Vilhelm Bjerknes, Jakob Bjerknes,
Halvor Solberg, and Tor Bergeron published their Norwegian
Cyclone model. This model proposed a life cycle for the
development of mid-latitude cyclones, and was mostly based
on surface observations. It became known as the American
Meteorological Society, cited 2020: Polar Front Theory.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Polar-front_theory.]"
class="glossaryLink">Polar Front Theory of a developing
wave cyclone. It was eventually modified and today provides a
way to describe the structure, American Meteorological Society,
cited 2019: Weather. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather, and evolution of a moving
cyclonic storm system in the mid-latitudes. First we will look at
how a mid-latitude cyclone develops at the surface, and then we
will look at how the surface evolution is affected by the winds
aloft.
ATMO 200 • 367
Satellite image of an extratropical cyclone over the UK (CC BY 2.0).
Cyclogenesis and Life Cycle
Following the Norwegian model, the development of a midlatitude cyclone begins along the American Meteorological
Society, cited 2020: Polar Front. Glossary of Meteorology.
368 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
[Available
online
at
http://glossary.ametsoc.org/wiki/
Polar_front.]" class="glossaryLink">polar front. Recall from
Chapter 11 that the American Meteorological Society, cited
2020: Polar Front. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front separates cold polar air from
warmer subtropical air at around 60° latitude. It is a semicontinuous boundary and mid-latitude cyclones form and move
along it as a series of waves. For this reason, a developing storm
is sometimes referred to as a wave cyclone. A full example for
cyclogensis and life cycle of a mid-latitude frontal cyclone in the
Northern Hemisphere will be discussed here.
This first figure shows a portion of the American Meteorological
Society, cited 2020: Polar Front. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Polar_front.]" class="glossaryLink">polar front as a stationary
front, with cold air to the north and warmer air to the south
flowing parallel to the American Meteorological Society, cited
2020: Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front in opposite directions. These winds
moving in opposite directions set up rotation, similar to how a
pen will turn if you place it between your hands and move them
in opposite directions.
ATMO 200 • 369
Part 1 of cyclogenesis: a stationary front with opposite moving winds on
either side (Public Domain).
Under the right conditions, a American Meteorological
Society, cited 2020: Frontal Wave. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Frontal_wave.]" class="glossaryLink">frontal wave will
begin to form along the American Meteorological Society, cited
2020: Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front,
with
a
cold
American
Meteorological Society, cited 2020: Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Front.]" class="glossaryLink">front pushing southward
and a American Meteorological Society, cited 2020: Warm
Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Warm_front.]"
class="glossaryLink">warm front moving northward. The
lowest pressure lies at the junction of the two fronts. The
southward-moving cold American Meteorological Society, cited
2020: Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front pushes warmer, less dense air
upward, while the American Meteorological Society, cited 2020:
370 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Warm Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Warm_front.]"
class="glossaryLink">warm front overruns and moves over the
colder air ahead of it. This creates rising motion in the column,
and a narrow band of American Meteorological Society, cited
2020: Precipitation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation forms. The surface low
pressure system is steered by winds aloft, typically moving
eastward or northeastward, and it gradually becomes a fullydeveloped mature cyclone 12 to 24 hours after its incipient
stage.
Part 2 of cyclogenesis: the formation of a frontal wave (Public Domain).
The central pressure lowers and the pressure gradient increases,
causing a stronger cyclonic (counterclockwise) flow inward
toward the low’s center. The American Meteorological Society,
cited 2020: Precipitation. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation band widens ahead of the
American Meteorological Society, cited 2020: Warm Front.
ATMO 200 • 371
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Warm_front.]"
class="glossaryLink">warm front, and narrows ahead of the
cold American Meteorological Society, cited 2020: Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front. The region of warmer air between
the cold and warm fronts here is called the warm sector. Here,
the American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather is generally partly cloudy, with
scattered showers possible if the air is conditionally unstable.
Where does the mid-latitude cyclone get its American
Meteorological Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy from? As the
warmer and colder air masses attempt to regain equilibrium,
warm air rises over the colder air, which transforms American
Meteorological Society, cited 2019: Potential energy. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Potential_energy.]" class="glossaryLink">potential energy
into kinetic (motion) American Meteorological Society, cited
2019: Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy. As warmer air rises, it condenses
into clouds, which release American Meteorological Society,
cited 2019: Latent heat. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Latent_heat.]"
372 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">latent heat American Meteorological
Society, cited 2019: Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy into the system. Near the surface,
winds converge inward toward the low’s center. Wind speeds
may increase as a result of a stronger American Meteorological
Society, cited 2020: Pressure Gradient Force. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Pressure-gradient_force.]" class="glossaryLink">pressure
gradient force near the center, increasing the American
Meteorological Society, cited 2019: Kinetic energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Kinetic_energy.]" class="glossaryLink">kinetic energy in
the system.
Part 3 of cyclogenesis: a newly developed cyclone (Public Domain).
As the cyclone moves eastward, the central pressure continues to
decrease and winds increase during its mature stage. The fastermoving cold American Meteorological Society, cited 2020:
Front. Glossary of Meteorology. [Available online at
ATMO 200 • 373
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front closes in on the American
Meteorological Society, cited 2020: Warm Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Warm_front.]"
class="glossaryLink">warm
front,
decreasing the size of the warm sector. In the Norwegian
cyclone model, the cold American Meteorological Society, cited
2020: Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front
overtakes
the
American
Meteorological Society, cited 2020: Warm Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Warm_front.]" class="glossaryLink">warm front and the
cyclone becomes occluded. Cloud cover and American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation cover a
wide area and the storm is usually most intense at this stage.
The point where the cold American Meteorological Society,
cited 2020: Front. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front, American Meteorological Society,
cited 2020: Warm Front. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Warm_front.]"
class="glossaryLink">warm
front,
and
American
Meteorological Society, cited 2020: Occluded Front. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Occluded_fronts.]" class="glossaryLink">occluded front
intersect is called the triple-point. Occasionally, a secondary low
374 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
may form at this triple point, move eastward, and intensify into
another cyclone.
Part 4 of cyclogenesis: formation of an occluded front and triple point
(Public Domain).
Eventually, as occlusion advances, the low pressure center will
begin to dissipate, because cold air exists on both sides of the
American Meteorological Society, cited 2020: Occluded Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Occluded_fronts.]"
class="glossaryLink">occluded front. The sector of warm,
rising air is removed from the center of the storm, so the storm
gets cut off from its primary American Meteorological Society,
cited 2019: Energy. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Energy.]"
ATMO 200 • 375
class="glossaryLink">energy supply. Eventually the old storm
dies out and gradually disappears.
Part 5 of cyclogenesis: continued formation of an occluded front and
dying out of the cyclone (Public Domain).
This sequence of a developing mid-latitude cyclone is similar to
a whirling, spinning eddy in a river that forms behind a stick or
log, moves along with the river, and quickly disappears further
downstream. This entire life cycle can last from several days to
a little more than a week.
Cyclones in various stages of development can be seen all at
once along the American Meteorological Society, cited 2020:
Polar Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front—this succession of storms is
known as a cyclone “family”. As mentioned before, some
376 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
cyclones form from dying previous cyclones and become a part
of the succession.
This American Meteorological Society, cited 2020: Polar Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Polar_front.]"
class="glossaryLink">polar front model of development for a
mid-latitude cyclone is rather simplified and, in fact, very few
storms follow this model exactly. However, it is a good
foundation for understanding storm structure.
Storm Tracks
The development of a mid-latitude cyclone is a process called
American Meteorological Society, cited 2020: Cyclogenesis.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Cyclogenesis.]"
class="glossaryLink">cyclogenesis. Certain regions in North
America are more favorable for American Meteorological
Society, cited 2020: Cyclogenesis. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Cyclogenesis.]" class="glossaryLink">cyclogenesis, including
the eastern slopes of mountain ranges like the Rockies and Sierra
Nevada, the Atlantic Ocean off the Carolina Coast, and the
Gulf of Mexico. When air flows westward across a north-south
extending mountain range, the air on the leeward (downwind)
side tends to have cyclonic curvature, which adds to the
development of a cyclone. This is called American
Meteorological Society, cited 2020: Lee Cyclogenesis.
Glossary
of
Meteorology.
[Available
online
at
ATMO 200 • 377
http://glossary.ametsoc.org/wiki/Lee_cyclogenesis.]"
class="glossaryLink">lee cyclogenesis, and cyclones that are
a result of this are often called lee-side lows/cyclones.
Cyclones may also develop near Cape Hatteras, North Carolina,
where warm moist air from the Gulf Stream can increase the
north-south air mass temperature/moisture contrast to the point
where American Meteorological Society, cited 2020:
Cyclogenesis. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Cyclogenesis.]"
class="glossaryLink">cyclogenesis might occur. These cyclones
are called northeasters (or nor’easters) and normally move
northeast along the Atlantic Coast. These storms can bring heavy
rain or snow and high winds to areas along the East Coast.
Typical cyclone storm tracks are named after the region in which
they form, like the Hatteras low, Alberta Clipper, or Colorado
low. Alberta clippers and Colorado lows form or re-develop on
the lee-side of the Rockies. Mid-latitude cyclones always move
toward the east due to the prevailing American Meteorological
Society, cited 2020: Westerlies. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Westerlies.]" class="glossaryLink">westerlies.
What influences the strength of a mid-latitude cyclone, and
determines how long it will persist? There are some surface
conditions that influence American Meteorological Society,
cited 2020: Cyclogenesis. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Cyclogenesis.]"
class="glossaryLink">cyclogenesis, but the real key to mid-
378 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
latitude cyclone development lies in the winds aloft. How are
mid-latitude cyclones influenced by upper-level flow?
Mid-latitude Cyclone in Three Dimensions
Developing surface lows are usually more intense with height
and appear on upper-level charts as a trough or a closed low.
However, the low in the upper-levels usually exists to the west
of the surface low (again, in the Northern Hemisphere). This is
a necessary condition for a low pressure system to continue to
develop and intensify. If the upper-level low were directly over
the surface low, the surface low would quickly dissipate. This is
because winds converge inward toward the low, but only at the
surface. This convergence at the surface causes the air mass to
“pile up” and air density to increase just above the surface low.
The increase in air mass causes surface pressures to rise, and
the low fills in and dissipates. How then do cyclones intensify
and develop? The air that piles up at the surface must have an
“exit path” out of the column so that the surface air pressure can
continue to decrease and the cyclone can strengthen. The vertical
structure of the atmosphere must allow for air to rise out of a
surface low pressure.
The following figure shows an idealized model of the vertical
structure of a cyclone and anticyclone in the Northern
Hemisphere. A surface low and a surface high are accompanied
by an upper level trough and ridge respectively.
ATMO 200 • 379
Diagram of the structure of a cyclone and anticyclone accompanied by a
trough and ridge (Image Created by Britt Seifert).
On the right hand side is a Northern Hemisphere frontal cyclone
with a warm and cold American Meteorological Society, cited
2020: Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front. The cold air behind the cold
American Meteorological Society, cited 2020: Front. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Front.]" class="glossaryLink">front at the surface also
extends upward aloft. Recall that the layer between two pressure
surfaces is thinner when the air temperature of the layer is cold
(more dense), and thicker when the layer is warm (less dense).
When pressure levels are packed closer together, pressure
decreases more rapidly with height in a column of cold air.
Because of this, low pressure is found aloft a body of cold
air, just as you find behind a cold American Meteorological
Society, cited 2020: Front. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front because the constant pressure
380 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
surfaces are squeezed closer to the Earth’s surface. Thus an
upper low is often found in the cold air aloft to the west of,
or behind, the surface low. The surface low tilts toward the
northwest moving up from the surface. Directly above the
surface low, the airflow spreads out and diverges. This allows
the converging surface air to rise and flow out of the air column
at the tropopause, reinforcing vertical motion. When the
divergence in the upper levels is stronger than convergence at
the surface, surface pressures will lower further, and the low
will intensify and deepen. Put another way: when air exits the
column more rapidly aloft than it enters the column at the
surface, then the amount of air in the column will reduce, the
surface pressure will lower, and the cyclone will intensify.
Notice that there is convergence directly aloft of the high
pressure system. Here, the air mass increases aloft and piles
up, while air flows clockwise (in the Northern Hemisphere) and
out of the anticyclone at the surface. If air were able to flow
freely out of the anticyclone, the air pressure would rapidly
drop and the anticyclone would dissipate. Therefore, to maintain
or strengthen the high pressure system, air has to continually
be added to the anticyclone. This happens when there is
convergence above a surface high. The air that piles up aloft
sinks in the column increasing surface pressure. If the
convergence aloft is stronger than the divergence at the surface
(more air is added than is removed), then the surface pressure
will increase.
Winds at the 500-mb pressure level tend to steer surface low
and high pressure systems. Generally speaking, surface storm
ATMO 200 • 381
systems tend to travel at about 16 knots in summer, and roughly
27 knots in winter. This is due to stronger jets and upper-level
flow in the winter, a result of stronger north-south temperature
differences.
Redistribution of Heat
Mid-latitude frontal cyclones are both a vital part of global
circulation and a result of global circulation. They’re also an
important pattern in the climatology of regions in the midlatitudes.
The temperature gradients that cause frontal cyclones form as
a result of the colliding surface air from the polar and Ferrel
cells. The strong temperature gradient with cold air from the
polar region and warm air from the tropics is the American
Meteorological Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy source that drives
the frontal storms. On the flip side of the same token, frontal
cyclones are a huge contributor to the redistribution of heat
globally. Warm air moving poleward in a American
Meteorological Society, cited 2020: Warm Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Warm_front.]" class="glossaryLink">warm front and cold
air moving equatorward in a cold American Meteorological
Society, cited 2020: Front. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front are later stabilized after the
temperature gradient balances itself by forcing the cold air aloft
382 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
(American Meteorological Society, cited 2020: Occluded Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Occluded_fronts.]"
class="glossaryLink">occluded front). Frontal cyclones are thus
both a result of and a contributor to global circulation and the
redistribution of heat from the equator to the poles.
The beginning of the chapter showed images of these storm
systems extending over scales the size of continents and landmasses. It also described how these storm systems last from
days to over a week. Instabilities along the American
Meteorological Society, cited 2020: Polar Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Polar_front.]" class="glossaryLink">polar front are always
growing and dying, and passing over fixed points on Earth. The
point is that if you live somewhere along the storm track in
the Northern or Southern hemisphere, in the wintertime, these
storm systems dictate your American Meteorological Society,
cited 2019: Weather. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather. For example, if you live in
Boston, Massachusetts, your winter American Meteorological
Society, cited 2019: Weather. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather may look something like this: a
few days of warm clear American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather, a quick change (passing cold
American Meteorological Society, cited 2020: Front. Glossary
ATMO 200 • 383
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Front.]" class="glossaryLink">front) resulting in a large
drop in temperatures and some heavy rain, then cold dry
American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather for a few days. This cold
American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather then transitions slowly to warm
by some light rain and warming temperatures (American
Meteorological Society, cited 2020: Warm Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Warm_front.]" class="glossaryLink">warm front). While
variable, this pattern repeats itself week after week. Depending
on the stage of the frontal storm as it passes over you, it may be
more or less severe, and you may receive more or less rain,
snow, or other wintery American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather. The American Meteorological
Society, cited 2020: Precipitation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Precipitation.]"
class="glossaryLink">precipitation
and
temperature variations resulting from frontal cyclones are an
important part of the climatology of mid-latitude American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather.
384 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Chapter 13: Questions to Consider
1.
Which American Meteorological
Society, cited 2020: Front. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front do midlatitude cyclones form and move along?
2.
How long does it take for a cyclone to
fully develop?
3.
What is the point where the cold
American Meteorological Society, cited
2020: Front. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front, American
Meteorological Society, cited 2020:
Warm Front. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Warm_front.]"
class="glossaryLink">warm front, and
American Meteorological Society, cited
2020: Occluded Front. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Occluded_fronts.]"
class="glossaryLink">occluded front
intersect called?
4.
True or False:
ATMO 200 • 385
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=392
5.
Put the steps of American
Meteorological Society, cited 2020:
Cyclogenesis. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Cyclogenesis.]"
class="glossaryLink">cyclogenesis in
the correct order from 1 to 5:
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=392
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=392
Chapter 14: Thunderstorm
Fundamentals
ALISON NUGENT AND SHINTARO RUSSELL
Learning Objectives
By the end of this chapter, you should be able to:
1.
Draw a diagram and label parts of a
American Meteorological Society, cited
2020: Thunderstorm. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Thunderstorm.]"
class="glossaryLink">thunderstorm
2.
Recognize that thunderstorms are
386
ATMO 200 • 387
sometimes part of a mid-latitude cyclone
3.
Describe the importance of updraft
separation from the downdraft and
American Meteorological Society, cited
2020: Precipitation. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Precipitation.]"
class="glossaryLink">precipitation
portion of a American Meteorological
Society, cited 2020: Thunderstorm.
Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm
4.
Identify a few types of thunderstorms
(airmass, MCC, American
Meteorological Society, cited 2020:
Squall Line. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Squall_line.]"
class="glossaryLink">squall line,
American Meteorological Society, cited
2020: Supercell. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Supercell.]"
class="glossaryLink">supercell, etc.)
5.
Discuss a few favorable conditions for
388 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
American Meteorological Society, cited
2020: Thunderstorm. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Thunderstorm.]"
class="glossaryLink">thunderstorm
formation
6.
Describe the importance of American
Meteorological Society, cited 2020: Wind
Shear. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Wind_shear.]"
class="glossaryLink">wind shear for
thunderstorms
7.
Identify LFC and EL on a skew-T as
well as CAPE and CIN and connect them
to American Meteorological Society,
cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Thunderstorm.]"
class="glossaryLink">thunderstorm
characteristics
8.
Compute updraft velocity from CAPE
ATMO 200 • 389
Lightning strikes the water from a thunderstorm in Northern Sicily (CC
BY-SA 2.0).
Introduction
Thunderstorms are deep convective clouds that have a large
vertical extent all the way from the boundary layer to the
tropopause. Thunderstorms often bring a variety of severe
American Meteorological Society, cited 2019: Weather.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather such as heavy rain, hail,
American Meteorological Society, cited 2020: Lightning.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Lightning.]"
class="glossaryLink">lightning, damaging winds, and,
390 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
occasionally, tornados. Thunderstorms are sometimes thought
to be synonymous with cumulonimbus clouds, though not all
cumulonimbus clouds are thunderstorms.
The primary American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy that drives thunderstorms is the
conversion of moist air into clouds and American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation, which
releases significant amounts of American Meteorological
Society, cited 2019: Latent heat. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Latent_heat.]" class="glossaryLink">latent heat in the
condensation process. Over land, thunderstorms occur most
frequently in the late afternoon and early evening. Storms are
most likely to occur soon after the warmest surface
temperatures, which helps to give warm air parcels the initial
buoyancy they need to begin to rise. Thunderstorms can last
anywhere from less than an hour to more than 12 hours. Because
of their rapid growth, relatively small (mesoscale) size, and
sensitivity to environmental conditions, it is difficult to predict
the exact time and location of a storm. Typically a forecast will
say there is a chance of thunderstorms over an area—meaning
that although conditions are conducive for a storm, the
forecasters don’t know exactly where or when it will occur.
ATMO 200 • 391
Thunderstorm Life Cycle
The three main stages of a thunderstorms are as follows:
1. Developing stage
2. Mature stage
3. Dissipating stage
In the developing stage, rising air called “updrafts” are
dominant in this stage and towering cumulus clouds form. This
type of cloud is also known as a cumulus congestus. Next is
the mature stage where the towering cumulus becomes a
cumulonimbus cloud featuring both updrafts and “downdrafts”
or sinking air. Downdrafts develop as a result of water droplets
falling to the ground as American Meteorological Society, cited
2020: Precipitation. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation (e.g., snow, rain, hail). The
falling drops drag air downward as they fall, creating a
downward current of air in the cumulonimbus cloud. Finally, in
the dissipating stage downdrafts are dominant and cut off the
updrafts needed for a cumulonimbus cloud to form and sustain
itself. As a result, the cloud dissipates.
If a cumulonimbus cloud has American Meteorological Society,
cited 2020: Thunder. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Thunder.]"
class="glossaryLink">thunder and American Meteorological
Society, cited 2020: Lightning. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
392 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Lightning.]" class="glossaryLink">lightning, it is called a
American
Meteorological
Society,
cited
2020:
Thunderstorm. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm.
American
Meteorological Society, cited 2020: Lightning. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Lightning.]" class="glossaryLink">Lightning is caused by
a charge separation in clouds. For this charge separation to
occur, a cloud must have ice crystals inside of it, which is why
thunderstorms are always mixed-phase or ice-phase clouds.
Formation and dissipation of a thunderstorm (Public Domain).
Airmass Thunderstorm
The American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm described above is called
an airmass American Meteorological Society, cited 2020:
ATMO 200 • 393
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm. In general, the ingredients
necessary for a American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm to occur are warm moist
air, some type of American Meteorological Society, cited
2020: Instability. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Instability.]"
class="glossaryLink">instability, and a trigger that can
initiate the storm system. Airmass thunderstorms typically occur
in environments with similar properties and little American
Meteorological Society, cited 2020: Wind Shear. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Wind_shear.]" class="glossaryLink">wind shear. Airmass
thunderstorms are the most benign types of thunderstorms and
typically short-lived, lasting less than a few hours.
The reason airmass thunderstorms are so short-lived and benign
is because the storm shuts off its ability to maintain itself. This
is due to two primary reasons:
1. Below cloud base the falling rain evaporates, cooling
the air in the boundary layer. The storm deprives
itself of the heat and moisture in the boundary-layer
air. Without the warm buoyant air to drive further
convection, the storm loses its strength and dissipates.
2. Rain falls within the updraft of the storm, reducing its
strength, again causing dissipation.
394 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Another diagram of an airmass American Meteorological
Society, cited 2020: Thunderstorm. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Thunderstorm.]" class="glossaryLink">thunderstorm or single
cell American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm is shown below. The initial
small cumulus cloud grows to a cumulus congestus, which has
mostly updrafts. It then grows further into a mature
cumulonimbus cloud with well defined updrafts and downdrafts.
The final stage is the dissipating stage, which has mostly
downdrafts.
Another diagram showing the typical cycle of a thunderstorm (CC
BY-SA 4.0).
The trigger for a American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm can be a warm humid air
mass heated from the bottom by daytime solar heating or forced
lifting by terrain (orographic thunderstorms). Collision of
airmasses can be another trigger, as you might find along a
American Meteorological Society, cited 2020: Warm Front.
ATMO 200 • 395
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Warm_front.]"
class="glossaryLink">warm
front,
cold
American
Meteorological Society, cited 2020: Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Front.]" class="glossaryLink">front, or dryline.
Thunderstorm Features
The main features of a mature American Meteorological Society,
cited 2020: Thunderstorm. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm
includes
updrafts,
downdrafts, overshooting tops, and an anvil. An American
Meteorological Society, cited 2020: Overshooting Top.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Overshooting_top.]"
class="glossaryLink">overshooting top occurs at the top of
the American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm. Overshooting tops are
dome-shaped as a result of strong updrafts in intense
thunderstorms reaching the tropopause and pushing upward into
the stable region. However, ultimately, because the tropopause
is so stable, the air gets pushed back downward and spread
laterally. This lateral spreading forms the cloud anvil at the
top of the American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
396 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">thunderstorm
boundary.
along
the
tropopause
Severe Thunderstorm
While basic airmass thunderstorms typically consist of one
“cell”, severe storms often contain multiple cells, consisting of
more than one rising thermal connected with the same storm
system. In a multicell storm, each cell is typically at a different
life cycle stage and they continue on one from another.
In order for a American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm to become severe, one
important additional ingredient is necessary. In addition to warm
moist air, some sort of American Meteorological Society, cited
2020: Instability. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Instability.]"
class="glossaryLink">instability, and a trigger, severe
thunderstorms need American Meteorological Society, cited
2020: Wind Shear. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear. American Meteorological
Society, cited 2020: Wind Shear. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Wind_shear.]" class="glossaryLink">Wind shear is defined as a
change in wind speed or direction with altitude. The reason
American Meteorological Society, cited 2020: Wind Shear.
Glossary
of
Meteorology.
[Available
online
at
ATMO 200 • 397
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear is important for severe storms
is easy; American Meteorological Society, cited 2020: Wind
Shear. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear helps to tilt a storm such that
the updraft and downdraft are displaced from one another.
Without the competition between upward moving air and
downward moving air, severe thunderstorms can strengthen and
last much much longer than airmass thunderstorms. Severe
thunderstorms come in many different forms. A few will be
discussed below.
Severe Thunderstorm Types
The three main types of thunderstorms include squall lines,
mesoscale convective complexes (MCCs), and supercells.
Squall Line
A American Meteorological Society, cited 2020: Squall Line.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Squall_line.]"
class="glossaryLink">squall line is a linear propagating
severe American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm. Squall lines consist of
a long, narrow line of merged thunderstorms, often triggered
by a cold American Meteorological Society, cited 2020: Front.
Glossary
of
Meteorology.
[Available
online
at
398 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front,
dry
line,
or
American
Meteorological Society, cited 2020: Gust Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Gust_front.]" class="glossaryLink">gust front. Squall lines
can be many hundreds of kilometers long, but their width is
usually much shorter, between 15 and 100 km. They can sustain
themselves from several hours to several days.
A American Meteorological Society, cited 2020: Squall Line.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Squall_line.]"
class="glossaryLink">squall line continues propagating with the
help of a American Meteorological Society, cited 2020: Gust
Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Gust_front.]"
class="glossaryLink">gust front. When rain falls into the drier
environment below the cloud base, it evaporates, cooling the air.
This cool air continues to move downward, sometimes with
strong force and reinforcement from a continuous downdraft.
When the cool downdraft hits the surface of Earth, it spreads
laterally, literally the same as a density current. Because it is
relatively cool, it acts like a mini cold American Meteorological
Society, cited 2020: Front. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front, pushing warmer air up over it as it
moves along. This upward push can initiate convection, and
continue fueling the larger storm system.
ATMO 200 • 399
Mesoscale Convective Complex
A American Meteorological Society, cited 2020: Mesoscale
Convective Complex. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Mesoscale_convective_complex.]"
class="glossaryLink">mesoscale convective complex (MCC) is
a type of severe storm that has a cloud shield (anvil) with a
diameter of at least 350 km, elliptical or circular shape, and lasts
between 6 and 12 hours. MCCs are huge storms that occur
multiple times per year especially in the central United States.
They are truly impressive masses of convection.
Supercell
A American Meteorological Society, cited 2020: Supercell.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Supercell.]"
class="glossaryLink">supercell is a special type of severe
storm that has rotation. Supercells form when the environment
has directional American Meteorological Society, cited 2020:
Wind Shear. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear causing updrafts within a
cloud to gain vorticity as they rise. While all severe storms
have a structure that separates the updraft from the downdraft,
American Meteorological Society, cited 2020: Supercell.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Supercell.]"
class="glossaryLink">supercell storms have a very clearly
defined structure, which allows them to sometimes form
400 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
tornados. As shown in the below image, supercells have
additional features like a American Meteorological Society,
cited 2020: Wall Cloud. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Wall_cloud.]"
class="glossaryLink">wall cloud and a American
Meteorological Society, cited 2020: Flanking Line. Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Flanking_line.]"
class="glossaryLink">flanking
line.
The
American
Meteorological Society, cited 2020: Wall Cloud. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Wall_cloud.]"
class="glossaryLink">wall
cloud
exists beneath the cloud base and near the border between the
downdraft of cold air and the lower-level inflow of warm humid
air. The American Meteorological Society, cited 2020: Flanking
Line. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Flanking_line.]"
class="glossaryLink">flanking line is the region of towering
cumulus that indicates extensive updrafts ahead of intense
thunderstorms.
ATMO 200 • 401
Diagram of a supercell thunderstorm with an anvil shape (CC BY-SA
3.0).
Here is a diagramed image of an actual American
Meteorological Society, cited 2020: Supercell. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Supercell.]" class="glossaryLink">supercell American
Meteorological Society, cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]" class="glossaryLink">thunderstorm that
created a tornado. Remember that supercells are rotating storm
systems that can produce tornados, but they don’t always. In
fact, very few supercells produce tornados, but the ones that do
get the most attention!
402 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
An image of a supercell cumulonimbus cloud (Public Domain).
Here is one last image showing the structure of a American
Meteorological Society, cited 2020: Supercell. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Supercell.]" class="glossaryLink">supercell from the top
view. The image shows the location of the American
Meteorological Society, cited 2020: Gust Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Gust_front.]" class="glossaryLink">gust front, the location
of various American Meteorological Society, cited 2020:
Precipitation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation types, and the location of
downdrafts within the storm system.
ATMO 200 • 403
A top view diagram of a supercell thunderstorm (Public Domain).
Thunderstorm Formation
Let’s review. The ingredients needed for American
Meteorological Society, cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm
formation include high humidity, conditional American
Meteorological Society, cited 2020: Instability. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Instability.]" class="glossaryLink">instability, and a
trigger that initiates rising air. A symmetric short-lived storm is
called an airmass American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm. When we add American
Meteorological Society, cited 2020: Wind Shear. Glossary of
404 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Wind_shear.]" class="glossaryLink">wind shear to an
airmass American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm, a severe American
Meteorological Society, cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]" class="glossaryLink">thunderstorm can
result. We discussed three types of severe thunderstorms
including squall lines, MCCs, and supercells. Supercells are a
type of severe storm with rotation resulting from directional
American Meteorological Society, cited 2020: Wind Shear.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear in the environment.
High humidity in the American Meteorological Society, cited
2019: Atmospheric boundary layer. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Atmospheric_boundary_layer.]"
class="glossaryLink">atmospheric boundary layer is required
for thunderstorms to occur. When water vapor condenses,
American Meteorological Society, cited 2019: Latent heat.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Latent_heat.]"
class="glossaryLink">latent heat is released. American
Meteorological Society, cited 2019: Latent heat. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Latent_heat.]" class="glossaryLink">Latent heat is the
ATMO 200 • 405
primary American Meteorological Society, cited 2019: Energy.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy source for thunderstorms. The
higher the humidity, the more American Meteorological Society,
cited 2019: Latent heat. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Latent_heat.]"
class="glossaryLink">latent heat is released and the stronger the
American Meteorological Society, cited 2020: Thunderstorm.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm becomes.
American Meteorological Society, cited 2020: Instability.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Instability.]"
class="glossaryLink">Instability is necessary for convection to
occur. The best case scenario is conditional American
Meteorological Society, cited 2020: Instability. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Instability.]" class="glossaryLink">instability, which often
occurs when cold air lies above warm moist air capped by a
temperature inversion. Cold air in the upper troposphere gives
greater buoyancy to the updraft of warmer air from below and,
therefore, a greater chance for strong thunderstorms to occur.
American Meteorological Society, cited 2020: Wind Shear.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">Wind shear is the change in wind speed
406 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
and or wind direction with height. In an environment with wind
but without American Meteorological Society, cited 2020: Wind
Shear. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear, thunderstorms would last
between 15 minutes and 1 hour as the American Meteorological
Society, cited 2020: Thunderstorm. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Thunderstorm.]"
class="glossaryLink">thunderstorm
and
boundary-layer air would remain together. In this scenario, an
airmass American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm dissipates after it deprives
itself of heat and moisture in the boundary-layer air. Strong
American Meteorological Society, cited 2020: Wind Shear.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear pushes thunderstorms away
from the depleted boundary-layer air and into areas with warm,
humid boundary-layer air. Thus, strong American
Meteorological Society, cited 2020: Wind Shear. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Wind_shear.]" class="glossaryLink">wind shear helps
thunderstorms sustain themselves for longer durations and to
grow stronger.
A trigger refers to any process that forces an American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
ATMO 200 • 407
wiki/Air_parcel.]" class="glossaryLink">air parcel to rise
through a cap of stable air and result in American
Meteorological Society, cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm
development. Triggers can be caused by frontal lifting,
orographic effects, or surface heating.
Atmospheric Instability and Thunderstorms
Because the potential for thunderstorms to develop depends on
atmospheric American Meteorological Society, cited 2019:
Stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability and layering, atmospheric
soundings (e.g., Skew-T log-P) are used by meteorologists to
help forecast storms. The soundings receive their data from the
rawinsonde balloon launches, aircraft observations, dropsondes,
satellites, or other meteorological data sources.
In atmospheric soundings, there are several labels indicating
atmospheric American Meteorological Society, cited 2019:
Stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]"
class="glossaryLink">stability. The labels are lifted
condensation level (LCL), American Meteorological Society,
cited 2019: Level of Free Convection. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Level_of_free_convection.]" class="glossaryLink">level of free
convection (LFC), American Meteorological Society, cited
408 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
2019: Level of neutral buoyancy. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Level_of_neutral_buoyancy.]"
class="glossaryLink">equilibrium level (EL), American
Meteorological Society, cited 2019: Convective Available
Potential Energy. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/
Convective_available_potential_energy.]"
class="glossaryLink">convective available potential energy
(CAPE), and American Meteorological Society, cited 2019:
Convective Inhibition. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Convective_inhibition.]"
class="glossaryLink">convective
inhibition (CIN).
Lifted Condensation Level (LCL) was discussed in previous
chapters. It is the altitude where the temperature cools to the
dew point temperature, resulting in American Meteorological
Society, cited 2019: Saturation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Saturation.]"
class="glossaryLink">saturation
and
condensation. The LCL is the location where the cloud base of
a American Meteorological Society, cited 2020: Thunderstorm.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm develops.
American Meteorological Society, cited 2019: Level of Free
Convection. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
ATMO 200 • 409
Level_of_free_convection.]" class="glossaryLink">Level of
Free Convection (LFC) is the height at which the environmental
temperature rate decreases faster than the American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel’s American
Meteorological Society, cited 2019: Moist adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Moist-adiabatic_lapse_rate.]"
class="glossaryLink">moist adiabatic lapse rate. This leads to
atmospheric American Meteorological Society, cited 2020:
Instability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Instability.]"
class="glossaryLink">instability. In an atmospheric sounding,
the LFC can be found where the American Meteorological
Society, cited 2019: Moist adiabatic lapse rate. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Moist-adiabatic_lapse_rate.]" class="glossaryLink">moist
adiabatic lapse rate of the rising parcel returns to the buoyant or
warmer side of the environmental temperature.
American Meteorological Society, cited 2019: Level of
neutral buoyancy. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Level_of_neutral_buoyancy.]"
class="glossaryLink">Equilibrium Level (EL) is the height
in the atmosphere where the temperature of the rising American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel is the same
410 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
as the temperature of its surroundings. The EL caps the
atmospheric American Meteorological Society, cited 2020:
Instability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Instability.]"
class="glossaryLink">instability. This is where the American
Meteorological Society, cited 2019: Moist adiabatic lapse rate.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Moist-adiabatic_lapse_rate.]"
class="glossaryLink">moist adiabatic lapse rate returns to the
negatively buoyant colder side of the environmental
temperature. This is also where the anvil top of the American
Meteorological Society, cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]" class="glossaryLink">thunderstorm is
typically located.
American Meteorological Society, cited 2019: Convective
Inhibition. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Convective_inhibition.]"
class="glossaryLink">Convective Inhibition (CIN) is the
amount of American Meteorological Society, cited 2019:
Energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy that prevents a rising American
Meteorological Society, cited 2019: Air parcel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Air_parcel.]" class="glossaryLink">air parcel from
reaching the American Meteorological Society, cited 2019:
Level of Free Convection. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
ATMO 200 • 411
Level_of_free_convection.]" class="glossaryLink">level of free
convection. On a thermodynamic diagram (Skew-T Log-P), it
is the negative area between the environmental American
Meteorological Society, cited 2019: Lapse rate. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Lapse_rate.]" class="glossaryLink">lapse rate and the
American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel American Meteorological
Society, cited 2019: Lapse rate. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Lapse_rate.]" class="glossaryLink">lapse rate. CIN must be
overcome for CAPE to be realized.
American Meteorological Society, cited 2019: Convective
Available Potential Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Convective_available_potential_energy.]"
class="glossaryLink">Convective
Available
Potential
Energy (CAPE) is the amount of American Meteorological
Society, cited 2019: Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Energy.]"
class="glossaryLink">energy an American Meteorological
Society, cited 2019: Air parcel. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Air_parcel.]" class="glossaryLink">air parcel would have if
lifted vertically through the atmosphere over a particular
distance. It is an indicator of atmospheric American
Meteorological Society, cited 2020: Instability. Glossary of
412 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Instability.]" class="glossaryLink">instability. On a
thermodynamic diagram (Skew-T Log-P), CAPE can be found
by the positive area between the environmental American
Meteorological Society, cited 2019: Lapse rate. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Lapse_rate.]" class="glossaryLink">lapse rate and the
American Meteorological Society, cited 2019: Air parcel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel American Meteorological
Society, cited 2019: Lapse rate. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Lapse_rate.]" class="glossaryLink">lapse rate. It is an
integrated measure of the total amount of buoyancy available
to a rising American Meteorological Society, cited 2019: Air
parcel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]"
class="glossaryLink">air parcel.
CAPE can be used to estimate the maximum updraft velocity in
thunderstorms.
While the above equation gives a good estimate, the updraft
velocity equation typically gives an unrealistically high value
as it ignores many important processes. For example, dry air
entrainment, liquid-water loading, and frictional drag are
ignored. Observational studies find that the typical updraft
velocity is roughly half of the maximum updraft velocity value.
ATMO 200 • 413
Thunderstorms are important features of Earth’s atmospheric
system. It is safe to say that there is always a American
Meteorological Society, cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm
occurring somewhere on Earth at all times. In many places,
thunderstorms provide needed rain and are an important piece
of the hydrological cycle. However, thunderstorms can also be
hazardous and impacts will be discussed in the following
chapter.
Chapter 14: Questions to Consider
1.
Label the typical lifecycle of a single
cell American Meteorological Society,
cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Thunderstorm.]"
class="glossaryLink">thunderstorm:
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
414 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=403
2.
Describe the importance of updraft
separation from the downdraft and
American Meteorological Society, cited
2020: Precipitation. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Precipitation.]"
class="glossaryLink">precipitation
portion of a American Meteorological
Society, cited 2020: Thunderstorm.
Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm.
3.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=403
4.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
ATMO 200 • 415
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=403
5.
With a CAPE of 1280 J/kg, calculate
the likely updraft velocity.
6.
Describe the importance of American
Meteorological Society, cited 2020: Wind
Shear. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Wind_shear.]"
class="glossaryLink">wind shear for
thunderstorms.
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=403
Chapter 15: Thunderstorm
Hazards
ALISON NUGENT AND DAVID DECOU
Learning Objectives
By the end of this chapter, you should be able to:
1.
Identify the different types of
American Meteorological Society, cited
2020: Precipitation. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Precipitation.]"
class="glossaryLink">precipitation (rain,
snow, American Meteorological Society,
cited 2020: Graupel. Glossary of
416
ATMO 200 • 417
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Graupel.]"
class="glossaryLink">graupel, hail, etc.)
2.
Describe how hail forms and why it is
difficult to predict
3.
Summarize how American
Meteorological Society, cited 2020:
Lightning. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Lightning.]"
class="glossaryLink">lightning occurs
with respect to American Meteorological
Society, cited 2020: Thunderstorm.
Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm ice
processes
4.
Discuss the various types of hazards
from thunderstorms (gust fronts and
outflow winds, American Meteorological
Society, cited 2020: Precipitation.
Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/
wiki/Precipitation.]"
class="glossaryLink">precipitation,
American Meteorological Society, cited
2020: Thunder. Glossary of Meteorology.
418 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
[Available online at
http://glossary.ametsoc.org/wiki/
Thunder.]"
class="glossaryLink">thunder and
American Meteorological Society, cited
2020: Lightning. Glossary of
Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Lightning.]"
class="glossaryLink">lightning,
tornadoes)
5.
Identify radar hook echoes that suggest
tornado hazards and discuss the process
of warning the public about a tornado
threat
Lightning interacts with a tornado in Texas (CC BY-SA 4.0).
ATMO 200 • 419
Introduction
A American Meteorological Society, cited 2020: Thunderstorm.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm is generally defined as any
storm that has American Meteorological Society, cited 2020:
Thunder. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunder.]"
class="glossaryLink">thunder and lighting. In fact, a
climatological American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm day is defined as any
day on which American Meteorological Society, cited 2020:
Thunder. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunder.]"
class="glossaryLink">thunder is audible from an observing
station, and American Meteorological Society, cited 2020:
Precipitation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation is not a required element.
However, thunderstorms almost always produce American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation along
with the required American Meteorological Society, cited 2020:
Thunder. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunder.]"
class="glossaryLink">thunder and American Meteorological
420 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Society, cited 2020: Lightning. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Lightning.]" class="glossaryLink">lightning, and many other
aspects that can be hazardous. Thunderstorms can produce
damaging surface winds, hail, heavy rain, and even extreme
phenomena like tornadoes. People are often most interested in
the damaging aspects of severe American Meteorological
Society, cited 2019: Weather. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather, as these most directly impact
human activity, and often produce beautiful and terrifying visual
displays. The hazardous American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather associated with thunderstorms
will be the topic of this chapter.
Precipitation and Hail
Thunderstorms consist of deep convective clouds that can
produce large raindrops (2–8 mm diameter) within 5–10 km
wide rain shafts that move horizontally across the ground with
American Meteorological Society, cited 2020: Precipitation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation that last from 1–20 minutes
over a fixed point. The rainfall rate may be heavy, as much as 10
to 1000 mm of rain per hour. The below photo depicts heavy rain
shafts produced by a American Meteorological Society, cited
2020: Supercell. Glossary of Meteorology. [Available online at
ATMO 200 • 421
http://glossary.ametsoc.org/wiki/Supercell.]"
class="glossaryLink">supercell American Meteorological
Society, cited 2020: Thunderstorm. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Thunderstorm.]"
class="glossaryLink">thunderstorm
in
Montana.
A supercell thunderstorm in Montana (CC BY 2.0).
The cloud tops of thunderstorms extend far up in the
troposphere. In the upper parts of the clouds, ice-phase processes
occur. Throughout the cloud, different sizes of ice crystals and
water droplets exist, and the heavier hydrometeors fall faster
than the smaller ones, sometimes colliding and collecting each
other. If these heavier ice particles fall through regions of
supercooled droplets (liquid water droplets below freezing), they
may grow through a process called riming. The liquid water
droplets instantly freeze and adhere on contact to the outside
surface of falling ice particles. This forms snow pellets called
American Meteorological Society, cited 2020: Graupel.
422 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Graupel.]"
class="glossaryLink">graupel, which are less than 5 mm in
diameter. Fallen American Meteorological Society, cited 2020:
Graupel. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Graupel.]"
class="glossaryLink">graupel is pictured below.
Graupel pellets on a car roof (CC BY 3.0).
Alternatively, smaller ice crystals that fall just below the 0°C
level may partially melt and stick to other partially melted ice
crystals through collision, causing them to grow through
American Meteorological Society, cited 2020: Aggregation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Aggregation.]"
class="glossaryLink">aggregation. These snow aggregates
can grow as large as 1 cm in diameter. Snow aggregates and
American Meteorological Society, cited 2020: Graupel.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Graupel.]"
class="glossaryLink">graupel can sometimes reach the ground
still frozen or partially frozen, even in the summer, if they fall
within cooler, saturated downdrafts. More frequently, these
ATMO 200 • 423
larger ice particles will melt completely into large raindrops
before hitting the ground.
American Meteorological Society, cited 2020: Hailstones.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Hailstones.]"
class="glossaryLink">Hailstones are irregularly shaped balls
of ice larger than 0.5 cm in diameter that are produced in a
cumulonimbus cloud—particularly in intense, severe
thunderstorms. Hail is formed when American Meteorological
Society, cited 2020: Graupel. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Graupel.]"
class="glossaryLink">graupel or other particles act as embryos
that grow through the American Meteorological Society, cited
2020:Accretion. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Accretion.]"
class="glossaryLink">accretion of supercooled liquid droplets
in a cloud. One raindrop takes about one million cloud droplets
to form, but a single hailstone may take as many as 10 billion
cloud droplets to form. A golf ball-sized hailstone, as pictured
below, must travel through the cloud for 5–10 minutes to be able
to grow to this size. Strong, violent updrafts within the storm are
able to carry smaller ice particles far above the freezing level
where collisions with supercooled droplets allow them to grow
into American Meteorological Society, cited 2020: Hailstones.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Hailstones.]"
class="glossaryLink">hailstones. Rotating updrafts, such as
those found in a American Meteorological Society, cited 2020:
Supercell. Glossary of Meteorology. [Available online at
424 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
http://glossary.ametsoc.org/wiki/Supercell.]"
class="glossaryLink">supercell, are capable of sweeping hail
horizontally through the storm, which may allow them to grow
much larger. As these growing ice particles pass through areas of
the cloud with high liquid water content, extra coatings of ice
form around them. Larger American Meteorological Society,
cited 2020: Hailstones. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Hailstones.]"
class="glossaryLink">hailstones may ascend very slowly in a
strong updraft, almost “floating” in place as many supercooled
water droplets continue to collide into them. When the hailstone
is carried away from the updraft, or if it gets too heavy, it will fall
because it is no longer supported by the rising air.
Just below the cloud, the American Meteorological Society,
cited 2020: Hailstones. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Hailstones.]"
class="glossaryLink">hailstones will begin to melt in the
warmer air. Small hail might completely melt before hitting
the ground, but in the strong updrafts of severe thunderstorms,
American Meteorological Society, cited 2020: Hailstones.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Hailstones.]"
class="glossaryLink">hailstones can grow large enough to reach
the surface before melting. Notice in the figure below that the
hailstone has concentric layers inside, similar to tree rings. The
concentric layers alternate clear and opaque ice. This is related
to the path that the hailstone takes when it passes through the
cloud, as well as the liquid water content of the different areas
the hailstone passes through. In areas with low liquid water
ATMO 200 • 425
content (dry growth regime), supercooled water droplets freeze
onto the hailstone instantly, which produces the milky opaque
ice that contains many air bubbles. In areas with high liquid
water content (wet growth regime), supercooled water droplets
collect very rapidly onto the hailstone. The transition from liquid
to ice releases American Meteorological Society, cited 2019:
Latent heat. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Latent_heat.]"
class="glossaryLink">latent heat, which keeps the surface
temperature of the hailstone at 0°C. As a result, supercooled
droplets will form a coat of water around the stone rather than
freezing instantly. This allows porous areas to be filled in as
the water coat freezes slowly and air bubbles escape from the
surface. This leaves a layer of clear ice around the hailstone.
Golf ball sized hail cut in half so the inside concentric layers can be
viewed (Public Domain).
Large hail can cause severe damage to crops, vegetation,
426 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
aircraft, roofs, windows, and create a traffic hazard if American
Meteorological Society, cited 2020: Hailstones. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Hailstones.]" class="glossaryLink">hailstones accumulate
on a roadway. Damage can be greater if the strong winds in
a American Meteorological Society, cited 2020: Thunderstorm.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm
cause
American
Meteorological Society, cited 2020: Hailstones. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Hailstones.]" class="glossaryLink">hailstones to move
horizontally as they fall. Larger American Meteorological
Society, cited 2020: Hailstones. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Hailstones.]" class="glossaryLink">hailstones have higher
terminal fall velocities and the potential to reach speeds higher
than 50 m·s-1 (112 mph).
Gust Fronts
Gusts fronts were already discussed in the prior chapter, but
we’ll include a section here as well. Recall that mature
thunderstorms contain both updrafts of warm, buoyant moist air
as well as downdrafts of chilled, dry, dense air. Downdrafts are
the result of a combination of evaporative cooling, American
Meteorological Society, cited 2020: Precipitation. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Precipitation.]" class="glossaryLink">precipitation drag,
and dry air entrainment into the cloud. With the onset of
ATMO 200 • 427
American Meteorological Society, cited 2020: Precipitation.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Precipitation.]"
class="glossaryLink">precipitation in a mature American
Meteorological Society, cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm,
there is often a downward rush of cold air. As this cold air hits
the surface, it spreads out along the surface in all directions. This
surface boundary between the cold horizontally rushing air and
the warmer air around it is called the American Meteorological
Society, cited 2020: Gust Front. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Gust_front.]" class="glossaryLink">gust front. Along this
boundary, winds rapidly change both in direction and speed.
The American Meteorological Society, cited 2020: Gust Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Gust_front.]"
class="glossaryLink">gust front is the leading edge of the
outflowing cold air at the surface, and resembles a mini cold
American Meteorological Society, cited 2020: Front. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Front.]" class="glossaryLink">front to surface observers.
If the American Meteorological Society, cited 2020: Gust Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Gust_front.]"
class="glossaryLink">gust front of an advancing American
Meteorological Society, cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]" class="glossaryLink">thunderstorm were
428 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
to pass by you, you would experience a sharp drop in
temperature accompanied by a shift in wind direction and an
increase in wind speeds, which may occasionally exceed 55
knots. The air is extremely turbulent along the leading edge of
the American Meteorological Society, cited 2020: Gust Front.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Gust_front.]"
class="glossaryLink">gust front. As the cold air pushes warm,
moist air upward along the boundary, a American
Meteorological Society, cited 2020: Shelf Cloud. Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Shelf_cloud.]"
class="glossaryLink">shelf cloud (or arcus cloud) may form,
as shown in the below figure.
A shelf cloud, or arcus cloud, on the edge of a gust front (CC BY-SA
3.0).
When the atmosphere is conditionally unstable, the American
ATMO 200 • 429
Meteorological Society, cited 2020: Gust Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Gust_front.]" class="glossaryLink">gust front may
produce additional multicell thunderstorms along its leading
edge as it forces warm, moist air upward. Each of these
additional thunderstorms will each have their own American
Meteorological Society, cited 2020: Gust Front. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Gust_front.]" class="glossaryLink">gust front, which may
merge together into a large American Meteorological Society,
cited 2020: Gust Front. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Gust_front.]"
class="glossaryLink">gust front known as an American
Meteorological Society, cited 2020: Outflow Boundary.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Outflow_boundary.]"
class="glossaryLink">outflow boundary.
Downbursts and Microbursts
Under a severe American Meteorological Society, cited 2020:
Thunderstorm. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm, the downdraft may become
narrow, intense, and localised. As it reaches the surface, it may
spread radially along the ground in a burst of wind—similar to
water rushing from a faucet and hitting the bottom of the sink.
This sort of downdraft is called a 4 km are termed macrobursts.
American Meteorological Society, cited 2020: Downburst.
430 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Downburst.]"
class="glossaryLink">downburst. A short 4 km are termed
macro-bursts.
American Meteorological Society, cited 2020: Downburst.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Downburst.]"
class="glossaryLink">downburst, with winds of 4 km or less,
is called a American Meteorological Society, cited 2020:
Microburst. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Microburst.]"
class="glossaryLink">microburst. Despite being small
spatially, a strong American Meteorological Society, cited 2020:
Microburst. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Microburst.]"
class="glossaryLink">microburst can contain powerful winds
up to 146 knots with the capacity to down trees and heavily
damage weak structures and boats. Some damage caused by
microbursts may have been incorrectly attributed to tornadoes.
In addition, American Meteorological Society, cited 2020: Wind
Shear. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear, turbulence, and gusts
associated with microbursts have been responsible for several
airplane crashes. Microbursts are often associated with severe
thunderstorms but have also been known to occur with ordinary
cell thunderstorms and even clouds that produce only scattered
showers.
ATMO 200 • 431
Cross-section of a microburst (Public Domain).
Lightning
American Meteorological Society, cited 2020: Lightning.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Lightning.]"
class="glossaryLink">Lightning is caused by a discharge of
electricity that typically occurs in mature thunderstorms.
American Meteorological Society, cited 2020: Lightning.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Lightning.]"
class="glossaryLink">Lightning can strike within the same
cloud, from one cloud to another, from a cloud to open air,
or from cloud to ground. Only about 20% of American
Meteorological Society, cited 2020: Lightning. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Lightning.]" class="glossaryLink">lightning strikes are
cloud-to-ground (CG) strikes, while the majority of strikes occur
within the cloud (CC). As a American Meteorological Society,
cited 2020: Lightning. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Lightning.]"
432 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">lightning stroke travels through the
atmosphere, it heats the surrounding air to a staggering
30,000°C, which is roughly five times hotter than the surface
of the sun. This incredible heating causes a sudden, explosive
expansion of the air, generating a shock wave in all directions
from the flash. This booms outward in the form of a sound
wave called American Meteorological Society, cited 2020:
Thunder. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunder.]"
class="glossaryLink">thunder.
Due to the speed that light travels, we see American
Meteorological Society, cited 2020: Lightning. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Lightning.]" class="glossaryLink">lightning instantly as it
happens. American Meteorological Society, cited 2020:
Thunder. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunder.]"
class="glossaryLink">Thunder travels much more slowly at
about 330 m·s-1 so it takes much longer to reach our ears. You
can estimate how distant a American Meteorological Society,
cited 2020: Lightning. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Lightning.]"
class="glossaryLink">lightning stroke is by counting the
seconds after you see the flash until you hear the American
Meteorological Society, cited 2020: Thunder. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunder.]" class="glossaryLink">thunder. It takes sound
about 5 seconds to travel one mile. If you hear American
Meteorological Society, cited 2020: Thunder. Glossary of
ATMO 200 • 433
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunder.]" class="glossaryLink">thunder roughly 10
seconds after you see the American Meteorological Society,
cited 2020: Lightning. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Lightning.]"
class="glossaryLink">lightning flash, then the strike occurred
about 2 miles away. However, if the American Meteorological
Society, cited 2020: Lightning. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Lightning.]" class="glossaryLink">lightning is too far, you may
not hear any American Meteorological Society, cited 2020:
Thunder. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunder.]"
class="glossaryLink">thunder, because sound waves attenuate
and bend as they travel through the atmosphere, especially
through layers of differing air temperature.
Lightning during a desert storm (CC 0).
434 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Why do we have American Meteorological Society, cited 2020:
Lightning. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Lightning.]"
class="glossaryLink">lightning? During ordinary fair American
Meteorological Society, cited 2019: Weather. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Weather.]" class="glossaryLink">weather conditions, the
earth’s surface has negative charge, while the upper atmosphere
is positively charged. American Meteorological Society, cited
2020: Lightning. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Lightning.]"
class="glossaryLink">Lightning requires differing regions of
opposite charge inside a cumulonimbus cloud. There are several
theories as to how this separation of charge may come about.
One theory is that clouds can become electrified when American
Meteorological Society, cited 2020: Graupel. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Graupel.]" class="glossaryLink">graupel and American
Meteorological Society, cited 2020: Hailstones. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Hailstones.]" class="glossaryLink">hailstones fall through
areas of supercooled droplets, causing the droplets to freeze
and release American Meteorological Society, cited 2019: Latent
heat. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Latent_heat.]"
class="glossaryLink">latent heat. This release of American
Meteorological Society, cited 2019: Latent heat. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Latent_heat.]" class="glossaryLink">latent heat keeps the
ATMO 200 • 435
hailstone surface warmer than the surrounding ice particles. A
net transfer of positive ions (molecules that carry a charge)
occurs from the warmer hailstone to the colder surrounding
ice crystals when they come in contact. Therefore, the larger,
warmer hailstone becomes negatively charged and the smaller,
cooler ice crystal becomes positively charged. This also happens
when supercooled liquid droplets freeze when they come into
contact with a warmer hailstone and little fragments of
positively charged ice break off. These tiny positively-charged
particles are easily swept to the upper regions of the cloud
through updrafts. The larger, heavier, negatively-charged
American Meteorological Society, cited 2020: Hailstones.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Hailstones.]"
class="glossaryLink">hailstones tend to remain suspended at
the same level in the updraft or fall to the lower parts of the
cloud. This causes the colder upper regions of the cloud to have a
net positive charge, while the middle level of the cloud has
negative charge. The lowest portion of the cloud generally has
a mixture of negative and positive charges, with a positively
charged region occasionally present in American Meteorological
Society, cited 2020: Precipitation. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Precipitation.]" class="glossaryLink">precipitation bands near
the melting level.
In short, thunderstorms must have ice phase processes to
produce American Meteorological Society, cited 2020: Thunder.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Thunder.]"
436 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">thunder and American Meteorological
Society, cited 2020: Lightning. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Lightning.]"
class="glossaryLink">lightning—American
Meteorological Society, cited 2020: Lightning. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Lightning.]" class="glossaryLink">lightning is produced
from a discharge of electricity and American Meteorological
Society, cited 2020: Thunder. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Thunder.]"
class="glossaryLink">thunder is simply produced as a result of
the American Meteorological Society, cited 2020: Lightning.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Lightning.]"
class="glossaryLink">lightning.
Tornadoes
Recall from the prior chapter that American Meteorological
Society, cited 2020: Supercell. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Supercell.]" class="glossaryLink">supercell storms can produce
tornados. A American Meteorological Society, cited 2020:
Supercell. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Supercell.]"
class="glossaryLink">supercell is a severe American
Meteorological Society, cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]" class="glossaryLink">thunderstorm with
rotation. Typically the rotation of a American Meteorological
ATMO 200 • 437
Society, cited 2020: Supercell. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Supercell.]"
class="glossaryLink">supercell
American
Meteorological Society, cited 2020: Thunderstorm. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm
comes from directional American Meteorological Society, cited
2020: Wind Shear. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear in the environment.
Occasionally a American Meteorological Society, cited 2020:
Supercell. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Supercell.]"
class="glossaryLink">supercell storm produces a tornado, a
region of rapidly rotating air in cyclostrophic balance. Recall
from Chapter 10 that cyclostrophic balance is a balance between
the American Meteorological Society, cited 2020: Pressure
Gradient Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Pressure-gradient_force.]"
class="glossaryLink">pressure gradient force and the American
Meteorological Society, cited 2020: Centrifugal Force. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Centrifugal_force.]"
class="glossaryLink">centrifugal
force. As the rotating air motion in a American Meteorological
Society, cited 2020: Supercell. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Supercell.]" class="glossaryLink">supercell is vertically
stretched, it narrows and speeds up. To picture this, imagine
a figure skater pulling their arms inward as they spin. Their
diameter narrows and their rotation speed increases. The same
438 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
physics—conservation of angular momentum—occurs as a
tornado forms.
An EF 4 Tornado in Marquette, Kansas (Public Domain).
Tornados are difficult to forecast well in advance. Typically a
Tornado Watch will be issued by a Storm Prediction Center
when all of the ingredients for a tornado are present in the
atmosphere. These ingredients are the same as for a severe
American Meteorological Society, cited 2020: Thunderstorm.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Thunderstorm.]"
class="glossaryLink">thunderstorm: warm moist air; American
Meteorological Society, cited 2020: Instability. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Instability.]" class="glossaryLink">instability (quantified
by large values of CAPE); potential for a trigger; and, of course,
directional American Meteorological Society, cited 2020: Wind
Shear. Glossary of Meteorology. [Available online at
ATMO 200 • 439
http://glossary.ametsoc.org/wiki/Wind_shear.]"
class="glossaryLink">wind shear to provide rotation. A
Tornado Warning is issued when a tornado is imminent or
occurring. For example, one may be issued when a funnel cloud
is spotted or when a “American Meteorological Society, cited
2020: Hook Echo. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Hook_echo.]"
class="glossaryLink">hook echo” is seen on radar. The issue
of forecasting tornados is called “nowcasting”. Although very
little advance notice can be given to the public, forecasters do
their best to provide as much notice as possible.
Hook echo in a radar image (Public Domain).
The above image shows a American Meteorological Society,
cited 2020: Hook Echo. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Hook_echo.]"
440 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
class="glossaryLink">hook echo from radar. The key feature is
the bottom left region of the storm where a region of rain can be
seen rapping around a region of dry inflow air. Right at the cusp
of the hook is where a tornado would form if it were occurring.
The strong rotation in a tornado creates this radar signature.
Tornado strength is categorized after they occur. A scale called
the Enhanced-Fujita scale is used to classify tornado strength
based on the amount of damage that was caused. While the wind
speed for each step in the EF scale is given, it is not measured.
The wind speed is determined afterwards by the scale of damage
and the winds required to cause the damage.
Enhanced Fujita Scale
Scale
Wind Speed (3
sec. gust, mph)
Damage
EF0
65-85
Minor
EF1
86-110
Moderate
EF2
111-135
Considerable
EF3
136-165
Severe
EF4
166-200
Extreme
EF5
>200
Massive/
Incredible
The central US is the world’s hot-spot for tornados, however,
tornados and tornado-like storm features can occur elsewhere.
A dust devil and a waterspout are tornado-like features that are
associated with American Meteorological Society, cited 2019:
Cumuliform. Glossary of Meteorology. [Available online at
ATMO 200 • 441
http://glossary.ametsoc.org/wiki/Cumuliform.]"
class="glossaryLink">cumuliform
and
sometimes
cumulonimbus clouds. However, dust devils and waterspouts
typically are not associated with a American Meteorological
Society, cited 2020: Supercell. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Supercell.]" class="glossaryLink">supercell. Waterspouts look
like tornados but form over water surfaces. Waterspouts are
often narrow, relatively short lived, and formed by the same
physics as a tornado—a rotating updraft with condensation in
the funnel cloud. Below is an image of a waterspout observed in
Hawaii off the coast of Oahu.
Waterspout spotted off O’ahu’s southern shore (Taken on May 2, 2011,
by Staff Sgt. Mike Meares).
Keeping Yourself Safe
From reading this storm hazards chapter, hopefully you’ve
442 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
learned that severe storms and storm hazards should be taken
seriously. Part of the issue with severe American Meteorological
Society, cited 2019: Weather. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">weather forecasting is that the impacts are
often extremely localized. It is difficult to know if you will be
in the exact location that hail will fall or a tornado will pass.
Oftentimes, you’ll simply be aware that the possibility is there
for storm impacts. Therefore, it is important to take precautions,
regardless of whether you are in the right place at the wrong
time.
For example, here are a few simple steps you can follow:
• “Turn around, don’t drown” is the saying from the
National American Meteorological Society, cited
2019: Weather. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/Weather.]"
class="glossaryLink">Weather Service. If you see
flowing water over a roadway, don’t drive over it!
• If there is a possibility for thunderstorms, don’t go
hiking or do outdoor activities near streams or rivers.
Flash flooding is a possibility.
• Similarly, if there is a possibility for thunderstorms,
don’t put yourself in an exposed outdoor location. As
soon as you hear American Meteorological Society,
cited 2020: Thunder. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Thunder.]" class="glossaryLink">thunder, a
American Meteorological Society, cited 2020:
ATMO 200 • 443
Thunderstorm. Glossary of Meteorology. [Available
online at http://glossary.ametsoc.org/wiki/
Thunderstorm.]" class="glossaryLink">thunderstorm
is close enough to be a hazard. Get yourself off of
that open sports field, beach, or stadium, and into a
shelter until it passes.
• If a tornado is approaching, the safest place is inside
the interior room in your house (unless you have a
tornado cellar). Get in your closet or your bathtub
with a mattress or something to cover you.
Depending on where you live in the world, some hazards are
more likely than others. Still, it is important to think in advance
about what you would do if you were in a severe storm situation.
Oftentimes there isn’t a lot of time to react.
Chapter 15: Questions to Consider
1.
Why does hail have concentric layers
that alternate clear and opaque ice?
2.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=417
444 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
3.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=417
4.
*FLASH* You saw a American
Meteorological Society, cited 2020:
Lightning. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Lightning.]"
class="glossaryLink">lightning stroke!
15 seconds go by… *BOOM* You heard
American Meteorological Society, cited
2020: Thunder. Glossary of Meteorology.
[Available online at
http://glossary.ametsoc.org/wiki/
Thunder.]"
class="glossaryLink">thunder! How far
away was the American Meteorological
Society, cited 2020: Lightning. Glossary
of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Lightning.]"
class="glossaryLink">lightning stroke?
5.
An interactive or media element has
been excluded from this version of the
ATMO 200 • 445
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=417
6.
Bob just finished reading this chapter
and wants to practice sheltering in the
event of a tornado warning. Can you help
him find the safest place to shelter in his
house? (Drag Bob to the correct room.)
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=417
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=417
Chapter 22: Atmospheric Optics
Learning Objectives
By the end of this chapter, you should be able to:
1.
Explain why the sky is blue and clouds
are white
2.
Discuss why lots of rainbows are seen
on Hawai’i
3.
Describe how one can observe a
rainbow
4.
Determine from visibility whether
objects are close or far away
446
ATMO 200 • 447
Rainbow over the Ala Wai Canal (Photo by Shintaro Russell).
Introduction
The color of our world is defined by the interaction of objects
with light. In fact, the reason we can see anything at all is due
to the interactions of objects with light. Light can be thought
of as both a particle and a wave. In this chapter, we’ll consider
light only as a wave defined by some wavelength on the
electromagnetic spectrum. Recall the electromagnetic American
Meteorological Society, cited 2019: Energy. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Energy.]" class="glossaryLink">energy spectrum from
Chapter 2 with the visible portion near the center between
ultraviolet and infrared:
448 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
The electromagnetic energy spectrum showing the wavelength of colors
(CC BY-NC-ND 2.0).
Notice that blue and purple colors have a relatively short
wavelength (~400 nm) and orange and red have a relatively long
wavelength (~700 nm). While slight, this difference makes a
large difference in the interaction of light with particles in our
atmosphere.
Before we begin, let’s define a few important facts that will help
us to understand the following material.
1. White light contains all colors. Light from the sun is
considered white light.
2. The color an object appears to be is due to reflection
of that color toward your eye. For example, a green
leaf looks green because it absorbs red and blue light,
but reflects the wavelength of light that is green.
Based on this simple description, we can say that the sky appears
to be blue because the molecules in the atmosphere reflect blue
ATMO 200 • 449
light. But why do they reflect blue light more than red light?
This has to do with the relative sizes of the molecules and the
wavelengths of light interacting with them.
In general, there are three different laws regarding wavelength
and interaction with particles. We’ll define λ as the wavelength
of light, and d as the diameter of the molecule or particle.
When the wavelength of light is much much larger than the
diameter of the molecule or particle, American Meteorological
Society, cited 2020: Rayleigh Scattering. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Rayleigh_scattering]"
class="glossaryLink">Rayleigh
scattering is dominant.
> d \end{align*}"
title="Rendered by QuickLaTeX.com">
When the wavelength of light is within an order or two of
magnitude as the diameter of the molecule or particle, American
Meteorological Society, cited 2020: Mie Scattering. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Mie_scattering.]" class="glossaryLink">Mie scattering is
dominant.
Finally, when the wavelength of light is much much smaller
than the diameter of the molecule or particle, American
Meteorological Society, cited 2020: Geometric Optics. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
450 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
wiki/Geometric_optics.]"
Optics is dominant.
class="glossaryLink">Geometric
Rayleigh Scattering
American Meteorological Society, cited 2020: Rayleigh
Scattering. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Rayleigh_scattering]"
class="glossaryLink">Rayleigh scattering is the reason the sky
is blue. American Meteorological Society, cited 2020: Rayleigh
Scattering. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Rayleigh_scattering]"
class="glossaryLink">Rayleigh scattering occurs when the
wavelength of visible light is much much larger than the
particles it is interacting with. Such is the case with the tiny
molecules in Earth’s atmosphere. The amount of scattering, “S”,
is proportional to:
Because this relationship is dependent on wavelength (λ),
shorter wavelengths will be scattered more. Purple and blue light
have a shorter wavelength than orange and red light and are
therefore scattered more, accounting for the blue color of Earth’s
atmosphere.
ATMO 200 • 451
Mie Scattering
American Meteorological Society, cited 2020: Mie Scattering.
Glossary
of
Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/Mie_scattering.]"
class="glossaryLink">Mie scattering is the reason clouds are
white. American Meteorological Society, cited 2020: Mie
Scattering. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Mie_scattering.]"
class="glossaryLink">Mie scattering occurs when the
wavelength of visible light is approximately the same as the
particle or droplet diameter (within a factor of 10 or 100). When
this is true, all wavelengths of visible light are scattered roughly
equally. In this case, the color illuminating is the same color
reflecting. In the case of clouds, they appear white because white
light is coming from the sun and illuminating them.
Geometric Optics
Finally, American Meteorological Society, cited 2020:
Geometric Optics. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Geometric_optics.]"
class="glossaryLink">geometric optics is responsible for
rainbows. A beam of light is called a ray. The tracing of the
ray path through a raindrop is called American Meteorological
Society, cited 2020: Geometric Optics. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Geometric_optics.]" class="glossaryLink">geometric optics and
helps us to understand how and why rainbows form. When
light comes in contact with a density interface, for example air
452 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
and water, it can reflect or refract. Refraction is responsible for
everyday conundrums like the following image.
Refraction of light at an air-water interface due to the difference in
density between the two media (CC BY-NC 2.0).
Refraction occurs when rays bend at a density interface.
ATMO 200 • 453
Reflection occurs when rays bounce back from an object. These
two phenomenon are responsible for why we can see
atmospheric optical phenomena such as rainbows, halos, and
sundogs. Here we will focus only on rainbows, the American
Meteorological Society, cited 2020: Geometric Optics. Glossary
of Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Geometric_optics.]"
class="glossaryLink">geometric
optics occurring in liquid rain drops.
Diagram showing angle of incidence, reflection, and refraction of a ray
of light (Public Domain).
As a light ray makes contact with an interface between two
media with different densities like air and water, some of the
incoming (incident) light will either refract from the air into the
water, reflect back into the air, or absorb into heat. In the case
of light interaction with a rain drop, we have a combination of
refraction and reflection. A single rainbow has two refractions
454 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
and one reflection, and the second rainbow seen in a double
rainbow has two refractions and two reflections.
The top right image of the following figure shows the interaction
of light with a rain drop responsible for a single rainbow. Light
comes in contact with a rain drop, refracts as it enters the rain
drop, reflects off the back side of the rain drop, and refracts
again as it exits the rain drop. The two refractions result in a
splitting of the colors of light, like a prism. This is because
refraction of light is dependent on wavelength; shorter
wavelengths refract more than longer wavelengths.
ATMO 200 • 455
The formation of a rainbow with light propagation. Legend: 1.)
Spherical droplet 2.) Places where internal reflection of the light occurs
3.) Primary rainbow 4.) Places where refraction of the light occurs 5.)
Secondary rainbow 6.) Incoming beams of white light 7.) Path of light
contributing to primary rainbow 8.) Path of light contributing to
secondary rainbow 9.) Observer 10.) Region forming the primary
rainbow 11.) Region forming the secondary rainbow 12.) Zone in the
atmosphere holding countless tiny spherical droplets (CC BY-SA 3.0).
Put concisely, a rainbow is an atmospheric phenomenon
456 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
displaying a spectrum of light as a result of refraction of light in
water droplets. This is possible because the wavelength of light
is much much smaller than the size of the rain drop.
Primary rainbows have red on the outside of the circle and
purple on the inside. There is always a 42° angle between the
incident light, the primary rainbow, and the observer. This
constant angle can be seen in the above figure. At a given time,
rainbows appear different to different observers because of
different interaction between incoming light rays and water
droplets. For instance, one observer may see a well-defined
rainbow in an area of large rain drops and bright sun. Another
observer in a half kilometer away with smaller rain drops may
see a partial rainbow.
Secondary rainbows are outer rainbows with red on the inside
of the circle and purple on the outside because of the double
reflection. Secondary rainbows occur at a 52° angle, hence
secondary rainbows are higher than primary rainbows. They’re
not as bright as primary rainbows, also because of the double
reflection. Triple rainbows are also possible and have been
observed, but are much less frequent.
ATMO 200 • 457
Double rainbow in a stormy sky. Notice how the colors are flipped in the
two rainbows, and how the secondary rainbow is not as bright as the
primary rainbow (Photo by Shintaro Russell).
How to see Rainbows
The best chance of seeing a rainbow is when there is rain and sun
low in the sky at the same time. An observer may see a rainbow
if the sun is at the observer’s back and they are facing towards
a region that is raining. As the sun lowers in the sky, the height
of the rainbow will increase because of the constant 42° angle.
Rainbows cannot be seen on the ground at noon because a 42°
angle is not possible. Rainbows appear as half arcs only because
the ground typically blocks the other half. Rainbows are frequent
in Hawai’i because of the abundant sunlight and frequent rain.
458 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Visibility
Visibility is the final topic on the interaction of light with
particles in Earth’s atmosphere covered in this chapter. In
general, the topic of visibility is simply a question of what can
be seen and how far away it is. When someone says that the
visibility today is 10 miles, it means that one can see objects 10
miles away.
In the Arctic, where the air is extremely dry and clean, visibility
of 100 miles (161 km) is common. On the other end, when you
cannot see objects 100 meters (~330 ft) away this is referred to
as zero visibility. As you may imagine, visibility is extremely
important for pilots and drivers. Fog in the atmosphere provides
the worst conditions for visibility.
Look at the following image of the Great Smoky Mountains.
The trees in the American Meteorological Society, cited 2020:
Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]"
class="glossaryLink">front of the image are nearly clear, but
with each additional ridge, the mountainside looks whiter and
whiter. We assume that each ridge has the same type of foliage as
the first. This tells us that objects further away are brighter.
ATMO 200 • 459
A photo from the Great Smoky Mountains National Park illustrating that
objects far away appear brighter (CC BY 2.0).
The reason for the change in brightness is particulate matter
in the atmosphere. This can come in the form of aerosols or
hydrometeors (cloud droplets, rain drops, snow flakes etc.). In
the case of the mountain image above, the Great Smoky
Mountains are known for emitting plentiful volatile organic
compounds (VOCs) that act as American Meteorological
Society, cited 2019: Cloud Condensation Nuclei. Glossary of
Meteorology. [Available online at http://glossary.ametsoc.org/
wiki/Cloud_condensation_nuclei.]"
class="glossaryLink">cloud condensation nuclei (CCN) and
produce clouds near ground level (i.e., low hanging fog). Both
the aerosols and the hydrometeors reduce visibility.
Many believe that a lack visibility is caused by objects blocking
your view. However, if that were the case, the background would
appear darker. Instead, a lack of visibility is caused by objects
460 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
scattering light towards your eye. With greater distances, your
eye has to look through a larger portion of the atmosphere and,
therefore, more opportunity for light to scatter towards you.
The scattered light is white because American Meteorological
Society, cited 2020: Mie Scattering. Glossary of Meteorology.
[Available
online
at
http://glossary.ametsoc.org/wiki/
Mie_scattering.]" class="glossaryLink">Mie scattering is
taking place.
Here is another image showing the same scene on a clear and
polluted day from the Sequoia and Kings Canyon National Park.
ATMO 200 • 461
A clear (top) and polluted (bottom) day at Sequoia & Kings Canyon
National Park (Public Domain).
This national park often has issues with pollution from the
central valley in California. On polluted days, more aerosols and
particulate matter are in the atmosphere, so more particles are
available to scatter light toward your eye. Scene details are lost
and the mountains in the distance can’t be seen at all. The visible
range is shorter on the bottom image as compared to the top.
462 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Chapter 22: Questions to Consider
1.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=463
2.
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=463
3.
Why is it not possible to see a rainbow
on the ground at noon?
4.
Drag the Sun, Rain Cloud, Rainbow,
and Bob to the correct positions in the
situation that Bob sees a rainbow when
looking East, assuming East is the right
side of the picture:
An interactive or media element has
been excluded from this version of the
text. You can view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=463
ATMO 200 • 463
5.
What causes low visibility? What type
of scattering is this an example of?
Selected Practice Question Answers:
An interactive or media element has been
excluded from this version of the text. You can
view it online here:
http://pressbooks-dev.oer.hawaii.edu/
atmo/?p=463
Glossary
Absolute humidity
The ratio of the mass of water vapor to the volume of air.
Absolutely stable
The environmental lapse rate is less than the moist
adiabatic lapse rate.
Absolutely unstable
The environmental lapse rate is greater than the dry
adiabatic lapse rate.
Accretion
In cloud physics, usually the growth of an ice hydrometeor
by collision with super-cooled cloud drops that freeze
wholly or partially upon contact.
American Meteorological Society, cited 2020:Accretion.
465
466 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Glossary of Meteorology. [Available
http://glossary.ametsoc.org/wiki/Accretion.]
online
at
Adiabatic processes
A process in which a system does not interact with its
surroundings by virtue of a temperature difference between
them.
American Meteorological Society, cited 2019: Adiabatic
processes. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Adiabatic_processes.]
Advection
The process of transport of an atmospheric property solely
by the mass motion (velocity field) of the atmosphere; also,
the rate of change of the value of the advected property at a
given point.
American Meteorological Society, cited 2020: Advection.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Advection.]
Aggregation
The process of clumping together of snow crystals
following collision as they fall to form snowflakes.
American
Meteorological
Society,
cited
2020:
Aggregation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Aggregation.]
ATMO 200 • 467
Air mass modification
The change of characteristics of an air mass as it moves
away from its region of origin.
American Meteorological Society, cited 2020: Air Mass
Modification. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Airmass_modification.]
Air parcel
An imaginary volume of air to which may be assigned any
or all of the basic dynamic and thermodynamic properties
of atmospheric air.
American Meteorological Society, cited 2019: Air parcel.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Air_parcel.]
Albedo
The ratio of reflected flux density to incident flux density,
referenced to some surface.
American Meteorological Society, cited 2019: Albedo.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Albedo.]
Anvil Cloud
The anvil-shaped cloud that comprises the upper portion of
mature cumulonimbus clouds; the popular name given to a
468 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
cumulonimbus capillatus cloud, particularly if it embodies
the supplementary feature incus.
American Meteorological Society, cited 2020: Anvil Cloud.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Anvil_cloud.]
Atmospheric boundary layer
The bottom layer of the troposphere that is in contact with
the surface of the earth.
American Meteorological Society, cited 2019: Atmospheric
boundary layer. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/
Atmospheric_boundary_layer.]
Atmospheric windows
A range of wavelengths over which there is relatively little
absorption of radiation by atmospheric gases. The major
windows are the visible window, from ∼0.3 to ∼0.9 μm;
the infrared window, from ∼8 to ∼13 μm; and the
microwave window, at wavelengths longer than ∼1 mm.
American Meteorological Society, cited 2019: Atmospheric
Window. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Atmospheric_window.]
Bergeron–Findeisen process
A theoretical explanation of the process by which
ATMO 200 • 469
precipitation particles may form within a mixed cloud
(composed of both ice crystals and liquid water drops).
American
Meteorological
Society,
cited
2020:
Bergeron–Findeisen process. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Bergeron-findeisen_process.]
Blackbody
A hypothetical body that cannot be excited to radiate by
an external source of electromagnetic radiation of any
frequency, direction, or state of polarization except in a
negligibly small set of directions around that of the source
radiation.
American Meteorological Society, cited 2019: Blackbody.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Blackbody.]
Centrifugal force
The apparent force in a rotating system, deflecting masses
radially outward from the axis of rotation, with magnitude
per unit mass (ω^2)(R), where ω is the angular speed of
rotation and R is the radius of curvature of the path.
American Meteorological Society, cited 2020: Centrifugal
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Centrifugal_force.]
470 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Clausius-Clapeyron equation
The differential equation relating pressure of a substance
to temperature in a system in which two phases of the
substance are in equilibrium.
Climate
The
slowly
varying
aspects
of
atmosphere–hydrosphere–land surface system.
the
American Meteorological Society, cited 2019: Climate.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Climate.]
Cloud Condensation Nuclei
Hygroscopic aerosol particles that can serve as nuclei of
atmospheric cloud droplets, that is, particles on which
water condenses (activates) at supersaturations typical of
atmospheric cloud formation (fraction of one to a few
percent, depending on cloud type).
American Meteorological Society, cited 2019: Cloud
Condensation Nuclei. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Cloud_condensation_nuclei.]
Collision–coalescence process
In cloud physics, the process produces precipitation by
ATMO 200 • 471
collision and coalescence between liquid particles (cloud
droplets, drizzle drops, and raindrops).
American Meteorological Society, cited 2020: Collisioncoalescence process. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Collisioncoalescence_process.]
Conditionally unstable
The environmental lapse rate is between the moist and dry
adiabatic lapse rates.
Conduction
Transport of energy (charge) solely as a consequence of
random motions of individual molecules (ions, electrons)
not moving together in coherent groups.
American Meteorological Society, cited 2019: Conduction.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Conduction.]
Contact freezing
When many freezing nuclei cause super-cooled liquid
droplets.
Contour
Generally, an outline or configuration of a body or surface.
Often, the term is used for one of a set of lines (contour
lines) drawn to represent the configuration of a surface.
472 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
American Meteorological Society, cited 2020: Contour.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Contours.]
Convection
The movement within a fluid due to the tendency of lower
density fluid to rise over higher density fluid, which sinks
due to the force of gravity resulting in heat transfer within
the fluid.
Convective Available Potential Energy
Also known as CAPE, is the maximum buoyancy of an
undiluted air parcel, related to the potential updraft strength
of thunderstorms.
American Meteorological Society, cited 2019: Convective
Available Potential Energy. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Convective_available_potential_energy.]
Convective inhibition
Also known as CIN, is the energy needed to lift an air
parcel upward adiabatically to the lifting condensation level
(LCL) and then as a pseudo-adiabatic process from the
LCL to its level of free convection (LFC).
American Meteorological Society, cited 2019: Convective
Inhibition. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Convective_inhibition.]
ATMO 200 • 473
Coriolis force
An apparent force on moving particles in a non-inertial
coordinate system, that is, the Coriolis acceleration as seen
in this (relative) system.
American Meteorological Society, cited 2020: Coriolis
Force. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Coriolis_force.]
Cumuliform
Like cumulus; generally descriptive of all clouds, the
principal characteristic of which is vertical development in
the form of rising mounds, domes, or towers.
American
Meteorological
Society,
cited
2019:
Cumuliform. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Cumuliform.]
Cyclogenesis
Any development or strengthening of cyclonic circulation
in the atmosphere; the opposite of cyclolysis.
American
Meteorological
Society,
cited
2020:
Cyclogenesis. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Cyclogenesis.]
Cyclostrophic wind
That horizontal wind velocity for which the centripetal
acceleration exactly balances the horizontal pressure force.
474 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
The cyclostrophic wind can be an approximation to the
real wind in the atmosphere only near the equator, where
the Coriolis acceleration is small; or in cases of very great
wind speed and curvature of the path (such as a tornado
or hurricane), so that the centripetal acceleration is the
dominant one.
American
Meteorological
Society,
cited
2020:
Cyclostrophic wind. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Cyclostrophic_wind.]
Dew point temperature
The temperature to which the air must be cooled to reach
saturation, without changing the moisture or air pressure.
Diabatic process
A thermodynamic change of state of a system in which the
system exchanges energy with its surroundings by virtue of
a temperature difference between them.
American Meteorological Society, cited 2019: Diabatic
process. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Diabatic_process.]
Doldrums
A nautical term for the equatorial trough, with special
reference to the light and variable nature of the winds.
ATMO 200 • 475
American Meteorological Society, cited 2020: Doldrums.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Doldrums.]
Downburst
An area of strong, often damaging, winds produced by
one or more convective downdrafts. Downbursts over
horizontal spatial scales ≤ 4 km are referred to as microbursts, whereas larger events with horizontal spatial scales
> 4 km are termed macro-bursts.
American Meteorological Society, cited 2020: Downburst.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Downburst.]
Downdraft
Sinking air.
Dropsondes
When the radiosonde packages are dropped from an
aircraft.
Dry adiabatic lapse rate
A process lapse rate of temperature, the rate of decrease
of temperature with height of a parcel of dry air lifted
by a reversible adiabatic process through an atmosphere
in hydrostatic equilibrium. The adiabatic lapse rate of
unsaturated air containing water vapor.
476 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
American Meteorological Society, cited 2019: Dryadiabatic lapse rate. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Dryadiabatic_lapse_rate.]
Eccentricity
The circularity of a planetary orbit.
Electromagnetic radiation
Energy propagated in the form of an advancing electric and
magnetic field disturbance.
American
Meteorological
Society,
cited
2019:
Electromagnetic radiation. Glossary of Meteorology.
[Available online at http://glossary.ametsoc.org/wiki/
Electromagnetic_radiation.]
Energy
A measurable physical quantity, with dimensions of mass
times velocity squared, that is conserved for an isolated
system. Energy of motion is kinetic energy; energy of
position is potential energy.
American Meteorological Society, cited 2019: Energy.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Energy.]
ATMO 200 • 477
Enhanced-Fujita scale
A scale used to classify tornado strength based on the
amount of damage that was caused.
Equation of State
Also known as the ideal gas law or the
Charles–Gay–Lussac law. An equation relating
temperature, pressure, and volume of a system in
thermodynamic equilibrium.
American Meteorological Society, cited 2019: Equation of
state. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Equation_of_state.]
Equilibrium level
The level at which an air parcel, rising or descending
adiabatically, attains the same density as its environment.
American Meteorological Society, cited 2019: Level of
neutral buoyancy. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Level_of_neutral_buoyancy.]
Eulerian framework
A fixed framework, relative to a single point on the Earth’s
surface.
478 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Ferrel cell
A zonally symmetric circulation that appears to be
thermally indirect (when viewed using height or pressure
as the vertical coordinate) first proposed by William Ferrel
in 1856 as the middle of three meridional cells in each
hemisphere.
American Meteorological Society, cited 2020: Ferrel cell.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Ferrel_cell.]
First law of thermodynamics
The total internal energy U of an isolated system is
constant. Energy cannot be created or destroyed.
American Meteorological Society, cited 2019: First law
of thermodynamics. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
First_law_of_thermodynamics.]
Flanking line
An organized lifting zone of cumulus and towering
cumulus clouds, connected to and extending outward from
the mature updraft tower of a supercell or strong multicell
convective storm.
American Meteorological Society, cited 2020: Flanking
Line. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Flanking_line.]
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Freezing nuclei
Ice nuclei that are effective at causing the freezing of supercooled liquid droplets.
Front
In meteorology, generally, the interface or transition zone
between two air masses of different density.
American Meteorological Society, cited 2020: Front.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Front.]
Frontal wave
A horizontal wave-like deformation of a front in the lower
levels, commonly associated with a maximum of cyclonic
circulation in the adjacent flow.
American Meteorological Society, cited 2020: Frontal
Wave. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Frontal_wave.]
Geometric optics
The application of ray tracing to explain scattering and
refractive effects by particles that are very large compared
with the wavelength of the radiation.
American Meteorological Society, cited 2020: Geometric
Optics. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Geometric_optics.]
480 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Geostrophic adjustment
The process by which an unbalanced atmospheric flow
field is modified to geostrophic equilibrium, generally by
a mutual adjustment of the atmospheric wind and pressure
fields depending on the initial horizontal scale of the
disturbance.
American Meteorological Society, cited 2020: Geostrophic
Adjustment. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Geostrophic_adjustment.]
Geostrophic balance
Describes a balance between Coriolis and horizontal
pressure-gradient forces.
American Meteorological Society, cited 2020: Geostrophic
Balance. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Geostrophic_balance.]
Gradient winds
Winds flowing along curved isobars.
Graupel
Heavily rimed snow particles, often called snow pellets;
often indistinguishable from very small soft hail except for
the size convention that hail must have a diameter greater
than 5 mm.
American Meteorological Society, cited 2020: Graupel.
ATMO 200 • 481
Glossary of Meteorology. [Available
http://glossary.ametsoc.org/wiki/Graupel.]
online
at
Greenhouse gases
Those gases, such as water vapor, carbon dioxide, ozone,
methane, nitrous oxide, and chlorofluorocarbons, that are
fairly transparent to the short wavelengths of solar radiation
but efficient at absorbing the longer wavelengths of the
infrared radiation emitted by the earth and atmosphere.
The trapping of heat by these gases controls the earth's
surface temperature despite their presence in only trace
concentrations in the atmosphere.
American Meteorological Society, cited 2019: Greenhouse
gases. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Greenhouse_gases.]
Gust front
The leading edge of a meso-scale pressure dome separating
the outflow air in a convective storm from the
environmental air.
American Meteorological Society, cited 2020: Gust Front.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Gust_front.]
Hadley cell
A direct thermally driven and zonally symmetric
circulation under the strong influence of the earth's rotation,
482 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
first proposed by George Hadley in 1735 as an explanation
for the trade winds.
American Meteorological Society, cited 2020: Hadley Cell.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Hadley_cell.]
Hailstones
A single unit of hail, ranging in size from that of a pea to,
on rare occasions, exceeding that of a grapefruit (i.e., from
5 mm to more than 15 cm in diameter).
American Meteorological Society, cited 2020: Hailstones.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Hailstones.]
Heat
The transfer of thermal energy due to the temperature
difference between two objects.
Heat capacity
The ratio of the energy or enthalpy absorbed (or released)
by a system to the corresponding temperature rise (or fall).
American Meteorological Society, cited 2019: Heat
capacity. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Heat_capacity.]
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Hook echo
A pendant, curve-shaped region of reflectivity caused when
precipitation is drawn into the cyclonic spiral of a mesocyclone.
American Meteorological Society, cited 2020: Hook Echo.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Hook_echo.]
Hydrostatic balance
Describes a balance between vertical pressure gradient and
buoyancy forces.
American Meteorological Society, cited 2019: Hydrostatic
balance. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Hydrostatic_balance.]
Hypsometric equation
An equation relating the thickness, h, between two isobaric
surfaces to the mean virtual temperature of the layer. The
hypsometric equation is derived from the hydrostatic
equation and the ideal gas law.
American Meteorological Society, cited 2019: Hypsometric
equation. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Hypsometric_equation.]
Ice nuclei
Any particle that serves as a nucleus leading to the
484 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
formation of ice crystals without regard to the particular
physical processes involved in the nucleation.
American Meteorological Society, cited 2020: Ice nucleus.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Ice_nucleus.]
Instability
A property of the steady state of a system such that certain
disturbances or perturbations introduced into the steady
state will increase in magnitude, the maximum perturbation
amplitude always remaining larger than the initial
amplitude.
American Meteorological Society, cited 2020: Instability.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Instability.]
Intertropical Convergence Zone (ITCZ)
The axis, or a portion thereof, of the broad trade-wind
current of the Tropics. This axis is the dividing line between
the southeast trades and the northeast trades (of the
Southern and Northern Hemispheres, respectively).
American Meteorological Society, cited 2020: Intertropical
Convergence Zone. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Intertropical_convergence_zone.]
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Isobars
A line of equal or constant pressure; an isopleth of pressure.
American Meteorological Society, cited 2020: Isobars.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Isobars.]
Isopleths
In common meteorological usage, a line of equal or
constant value of a given quantity, with respect to either
space or time.
American Meteorological Society, cited 2020: Isopleths.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Isopleths.]
Isotherms
A line of equal or constant temperature.
American Meteorological Society, cited 2020: Isotherms.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Isotherms.]
Jet stream
Relatively strong winds concentrated within a narrow
stream in the atmosphere.
American Meteorological Society, cited 2020: Jet Steam.
486 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Glossary of Meteorology. [Available
http://glossary.ametsoc.org/wiki/Jet_stream.]
online
at
Kinetic energy
The energy that a body possesses as a consequence of its
motion, defined as one- half the product of its mass and the
square of its speed.
American Meteorological Society, cited 2019: Kinetic
energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Kinetic_energy.]
Lagrangian framework
A framework that is constantly moving and travels with the
air.
Lapse rate
The decrease of an atmospheric variable with height, the
variable being temperature, unless otherwise specified.
American Meteorological Society, cited 2019: Lapse rate.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Lapse_rate.]
Latent heat
The specific enthalpy difference between two phases of a
substance at the same temperature.
American Meteorological Society, cited 2019: Latent heat.
ATMO 200 • 487
Glossary of Meteorology. [Available online
http://glossary.ametsoc.org/wiki/Latent_heat.]
at
Lee Cyclogenesis
The synoptic-scale development of an atmospheric
cyclonic circulation on the downwind side of a mountain
range.
American Meteorological Society, cited 2020: Lee
Cyclogenesis. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Lee_cyclogenesis.]
Level of free convection
The level at which a parcel of air lifted dry-adiabatically
until saturated and saturation-adiabatically thereafter would
first become warmer than its surroundings in a
conditionally unstable atmosphere.
American Meteorological Society, cited 2019: Level of
Free Convection. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Level_of_free_convection.]
Lifting Condensation Level
The level at which a parcel of moist air lifted dryadiabatically would become saturated. This is where clouds
form.
American Meteorological Society, cited 2019: Lifting
488 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Condensation Level. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Lifting_condensation_level.]
Lightning
Lightning is a transient, high-current electric discharge with
path lengths measured in kilometers.
American Meteorological Society, cited 2020: Lightning.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Lightning.]
Mesoscale convective complex
A subset of mesoscale convective systems (MCS) that
exhibit a large, circular (as observed by satellite), longlived, cold cloud shield.
American Meteorological Society, cited 2020: Mesoscale
Convective Complex. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Mesoscale_convective_complex.]
Microburst
A convective downdraft (downburst) that covers an area
less than 4 km along a side with peak winds that last 2–5
minutes.
American Meteorological Society, cited 2020: Microburst.
ATMO 200 • 489
Glossary of Meteorology. [Available
http://glossary.ametsoc.org/wiki/Microburst.]
online
at
Mie scattering
Scattering of electromagnetic waves by homogeneous
spheres of arbitrary size, named after Gustav Mie
(1868–1957), whose theory of 1908 explains the process.
American Meteorological Society, cited 2020: Mie
Scattering. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Mie_scattering.]
Mixing ratio
The ratio of the mass of water vapor to the mass of dry air.
Moist adiabatic lapse rate
The rate of decrease of temperature with height along a
moist adiabat.
American Meteorological Society, cited 2019: Moist
adiabatic lapse rate. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/Moistadiabatic_lapse_rate.]
Neutral stability
A condition of a system for which a small perturbation of a
parcel of the system causes it to neither depart from its new
position nor return to its previous one.
490 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
American Meteorological Society, cited 2019: Neutral
stability. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Neutral_stability.]
Obliquity
The degree of tilt in the axis of rotation.
Occluded front
A front that forms as a cyclone moves deeper into colder
air.
American Meteorological Society, cited 2020: Occluded
Front. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Occluded_fronts.]
Outflow boundary
A surface boundary formed by the horizontal spreading of
thunderstorm-cooled air.
American Meteorological Society, cited 2020: Outflow
Boundary. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Outflow_boundary.]
Overshooting top
A domelike protrusion above a cumulonimbus anvil,
representing the intrusion of an updraft through its
equilibrium level (level of neutral buoyancy).
American
Meteorological
Society,
cited
2020:
ATMO 200 • 491
Overshooting Top. Glossary of Meteorology. [Available
online
at
http://glossary.ametsoc.org/wiki/
Overshooting_top.]
Planck's radiation law
The distribution law of photon energies for radiation in
equilibrium with matter at absolute temperature T.
American Meteorological Society, cited 2019: Planck’s
radiation law. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/
Planck%27s_radiation_law.]
Polar cell
A weak meridional circulation in the high-latitude
troposphere characterized by ascending motion in the subpolar latitudes (50°–70°), descending motion over the pole,
poleward motion aloft, and equatorward motion near the
surface.
American Meteorological Society, cited 2020: Polar Cell.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Polar_cell.]
Polar Easterlies
Air typically flowing from the northeast while in the
Antarctic, air flowing from the southeast.
492 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Polar front
According to the polar-front theory, the semi-permanent,
semi-continuous front separating air masses of tropical and
polar origin.
American Meteorological Society, cited 2020: Polar Front.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Polar_front.]
Polar Front theory
A theory originated by the Scandinavian school of
meteorologists whereby a polar front, separating air masses
of polar and tropical origin, gives rise to cyclonic
disturbances that intensify and travel along the front,
passing through various phases of a characteristic life
history.
American Meteorological Society, cited 2020: Polar Front
Theory. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Polar-front_theory.]
Polar jet stream
Relatively strong winds concentrated within a narrow
stream in the atmosphere. The polar-front jet stream is
associated with the polar front of middle and upper-middle
latitudes.
American Meteorological Society, cited 2020: Polar Jet
ATMO 200 • 493
Stream. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Polar-front_jet_stream.]
Potential energy
The energy a system has by virtue of its position; the
negative of the work done in taking a system from a
reference configuration, where the potential energy is
assigned the value zero, to a given configuration, with no
change in kinetic energy of the system.
American Meteorological Society, cited 2019: Potential
energy. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Potential_energy.]
Potential temperature
The temperature that an unsaturated parcel of dry air would
have if brought adiabatically and reversibly from its initial
state to a standard pressure, p₀, typically 100 kPa.
American Meteorological Society, cited 2019: Potential
temperature. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Potential_temperature.]
Precession
The wobble in the rotational axis of a planet that slowly
traces out a cone.
494 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
Precipitation
All liquid or solid phase aqueous particles that originate in
the atmosphere and fall to the earth's surface.
American
Meteorological
Society,
cited
2020:
Precipitation. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Precipitation.]
Pressure Gradient force
The force due to differences of pressure within a fluid mass.
American Meteorological Society, cited 2020: Pressure
Gradient Force. Glossary of Meteorology. [Available online
at
http://glossary.ametsoc.org/wiki/Pressuregradient_force.]
Radiation
The process by which electromagnetic radiation is
propagated through free space.
American Meteorological Society, cited 2019: Radiation.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiation.]
Radiosonde
An expendable meteorological instrument package, often
borne aloft by a free-flight balloon, that measures, from
the surface to the stratosphere, the vertical profiles of
atmospheric variables and transmits the data via radio to
ATMO 200 • 495
a ground receiving system. Radiosondes typically measure
temperature, humidity, and, in many cases, pressure.
American Meteorological Society, cited 2020: Radiosonde.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Radiosonde.]
Rawinsondes
Radiosondes that also infer wind data at different heights.
Rayleigh scattering
Approximate theory for electromagnetic scattering by small
particles named for Lord Rayleigh (John William Strutt,
1842–1919), who in 1871 showed that the blue color of
the clear sky is explained by the scattering of light by
molecules in the atmosphere.
American Meteorological Society, cited 2020: Rayleigh
Scattering. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Rayleigh_scattering]
Relative humidity
The ratio of the amount of water vapor present in the air to
the maximum amount of water vapor needed for saturation
at a certain pressure and temperature.
Saturation
The condition in which vapor pressure is equal to the
496 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
equilibrium vapor pressure over a plane surface of pure
liquid water, or sometimes ice.
American Meteorological Society, cited 2019: Saturation.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Saturation.]
Shelf cloud
A low-level, horizontal, wedge-shaped arcus cloud
associated with a convective storm's gust front (or
occasionally a cold front).
American Meteorological Society, cited 2020: Shelf Cloud.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Shelf_cloud.]
Snowflake
Colloquially an ice crystal, or more commonly an
aggregation of many crystals that falls from a cloud.
American Meteorological Society, cited 2020: Snowflake.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Snowflakes.]
Sounding
A vertical profile of the environmental lapse rate by
releasing a radiosonde attached to a weather balloon.
ATMO 200 • 497
Specific heat
The heat capacity of a system divided by its mass.
American Meteorological Society, cited 2019: Specific
heat. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Specific_heat.]
Specific humidity
The ratio of the mass of water vapor to the total mass of air
(dry air and water vapor combined).
Spontaneous freezing
When liquid water droplets freeze without any sort of
nucleus.
Squall line
A line of active deep moist convection frequently
associated with thunder, either continuous or with breaks,
including contiguous precipitation areas.
American Meteorological Society, cited 2020: Squall Line.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Squall_line.]
Stability
The characteristic of a system if sufficiently small
disturbances have only small effects, either decreasing in
amplitude or oscillating periodically; it is asymptotically
498 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
stable if the effect of small disturbances vanishes for long
time periods.
American Meteorological Society, cited 2019: Stability.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stability.]
Stationary front
A type of frontal system that are almost stationary with the
winds flowing nearly parallel and from the opposite paths
in each side separated by the front.
Steady-state
A fluid motion in which the velocities at every point of the
field are independent of time; streamlines and trajectories
are identical.
American Meteorological Society, cited 2019: Steady state.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Steady_state.]
Stefan-Boltzmann Law
One of the radiation laws, which states that the amount of
energy radiated per unit time from a unit surface area of an
ideal blackbody is proportional to the fourth power of the
absolute temperature of the blackbody.
American Meteorological Society, cited 2019: StefanBoltzmann Law. Glossary of Meteorology. [Available
ATMO 200 • 499
online
at
http://glossary.ametsoc.org/wiki/Stefanboltzmann_law.]
Stoke’s Drag Law
Equation to find the terminal velocity of a falling cloud
droplet.
Stratiform
Descriptive of clouds of extensive horizontal development,
as contrasted to the vertically developed cumuliform types.
American Meteorological Society, cited 2019: Stratiform.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Stratiform.]
Subpolar Low
A band of low pressure located, in the mean, between 50°
and 70° latitude.
American Meteorological Society, cited 2020: Subpolar
Low. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/
Subpolar_low_pressure_belt]
Subtropical Highs
These highs appear as centers of action on mean charts of
sea level pressure, generally between 20° and 40° latitude.
They lie over the oceans and are best developed in the
summer season.
500 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
American Meteorological Society, cited 2020: Subtropical
Highs. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Subtropical_high.]
Super-cooled water
Liquid water that exists below the freezing point of water
(below 0°C).
Supercell
An often dangerous convective storm that consists
primarily of a single, quasi-steady rotating updraft, which
persists for a period of time much longer than it takes an
air parcel to rise from the base of the updraft to its summit
(often much longer than 10–20 min).
American Meteorological Society, cited 2020: Supercell.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Supercell.]
Synoptic
In meteorology, this term has become somewhat
specialized in referring to the use of meteorological data
obtained simultaneously over a wide area for the purpose
of presenting a comprehensive and nearly instantaneous
picture of the state of the atmosphere.
American Meteorological Society, cited 2020: Synoptic.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Synoptic.]
ATMO 200 • 501
Terminal velocity
The particular falling speed, for any given object moving
through a fluid medium of specified physical properties, at
which the drag forces and buoyant forces exerted by the
fluid on the object just equal the gravitational force acting
on the object.
American Meteorological Society, cited 2020: Terminal
Fall Velocity. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Terminal_fall_velocity.]
Thermal energy
A form of energy transferred between systems, existing
only in the process of transfer. Also the same as enthalpy.
American Meteorological Society, cited 2019: Heat.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Heat.]
Thunder
The sound emitted by rapidly expanding gases along the
channel of a lightning discharge.
American Meteorological Society, cited 2020: Thunder.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Thunder.]
Thunderstorm
In general, a local storm, invariably produced by a
502 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
cumulonimbus cloud and always accompanied by lightning
and thunder, usually with strong gusts of wind, heavy rain,
and sometimes with hail.
American
Meteorological
Society,
cited
2020:
Thunderstorm. Glossary of Meteorology. [Available online
at http://glossary.ametsoc.org/wiki/Thunderstorm.]
Trade winds
The wind system, occupying most of the Tropics, that
blows from the subtropical highs toward the equatorial
trough; a major component of the general circulation of the
atmosphere.
American Meteorological Society, cited 2020: Trade
Winds. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Trade_winds.]
Turbulent drag
The relationship between wind speed and force caused by
the wind against objects or along surfaces.
American Meteorological Society, cited 2020: Turbulent
Drag. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Drag_law.]
Updraft
Rising air.
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Vapor pressure
The pressure exerted by the molecules of a given vapor.
American Meteorological Society, cited 2019: Vapor
pressure. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Vapor_pressure.]
Virtual temperature
Also called Density temperature. The temperature that dry
dry air would have if its pressure and density were equal to
those of a given sample of moist air.
American Meteorological Society, cited 2019: Virtual
temperature. Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Virtual_temperature.]
Wall cloud
A local, often abrupt lowering from a cumulonimbus cloud
base into a low-hanging accessory cloud, normally a
kilometer or more in diameter.
American Meteorological Society, cited 2020: Wall Cloud.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Wall_cloud.]
Warm front
Any non-occluded front, or portion thereof, that moves in
such a way that warmer air replaces cold air.
504 • ALISON NUGENT, DAVID DECOU, AND SHINTARO RUSSELL
American Meteorological Society, cited 2020: Warm Front.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Warm_front.]
Warm sector
The region of warmer air between the cold and warm
fronts.
Weather
The state of the atmosphere, mainly with respect to its
effects upon life and human activities. As distinguished
from climate, weather consists of the short-term (minutes
to days) variations in the atmosphere. Popularly, weather is
thought of in terms of temperature, humidity, precipitation,
cloudiness, visibility, and wind.
American Meteorological Society, cited 2019: Weather.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Weather.]
Westerlies
Specifically, the dominant west-to-east motion of the
atmosphere, centered over the middle latitudes of both
hemispheres.
American Meteorological Society, cited 2020: Westerlies.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Westerlies.]
ATMO 200 • 505
Wet-Bulb temperature
The lowest temperature that can be achieved if water
evaporates within the air.
Wien’s Law
A radiation law that is used to relate the wavelength of
maximum emission from a blackbody inversely to its
absolute temperature.
American Meteorological Society, cited 2019: Wien’s law.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Wien%27s_law.]
Wind shear
The local variation of the wind vector or any of its
components in a given direction.
American Meteorological Society, cited 2020: Wind Shear.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Wind_shear.]
Work
A form of energy arising from the motion of a system
against a force, existing only in the process of energy
conversion.
American Meteorological Society, cited 2019: Work.
Glossary of Meteorology. [Available online at
http://glossary.ametsoc.org/wiki/Work.]